diff --git a/mkdocs.yml b/mkdocs.yml
index 2e5c7b3576..c21722567c 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -46,6 +46,7 @@ nav:
           - Quantitative MRI: appendices/qmri.md
           - Arterial Spin Labeling: appendices/arterial-spin-labeling.md
           - Cross modality correspondence: appendices/cross-modality-correspondence.md
+          - Phenotypic data guidelines: appendices/phenotype.md
       - Changelog: CHANGES.md
   - The BIDS Starter Kit:
       - Website: https://bids-standard.github.io/bids-starter-kit/
diff --git a/src/appendices/phenotype.md b/src/appendices/phenotype.md
new file mode 100644
index 0000000000..66a96cd6b4
--- /dev/null
+++ b/src/appendices/phenotype.md
@@ -0,0 +1,335 @@
+# Tabular phenotypic data guidelines
+
+This appendix is a collection of guidelines and examples for creating well-organized aggregated tabular phenotypic data.
+
+## Guidelines
+
+These guidelines are all **RECOMMENDED** when preparing
+tabular phenotypic data like the
+participants file, sessions file, demographics file,
+or phenotypic and assessment data.
+The language below uses REQUIRED, MUST, and others to imply
+these are the requirements for these **RECOMMENDED** guidelines.
+
+### 1. Always pair tabular data with data dictionaries
+
+Tabular phenotypic data MUST be prepared as one pair of a tabular file
+in tab-separated value (TSV) format and a corresponding data dictionary
+in JavaScript Object Notation (JSON) format.
+
+### 2. Aggregate data across sessions
+
+Aggregation refers to the contents of the TSV file. It is REQUIRED
+to collect all participant data into one TSV per tabular phenotypic file.
+
+### 3. Ensure minimal annotation for phenotypic and assessment data
+
+In phenotypic and assessment data each measurement tool has an independent
+aggregated data TSV file in which the user collects all subjects, sessions,
+and/or runs of data as one entry per row (with a row defined by
+the smallest unit of acquisition). In other words:
+
+1.  Each row MUST start with `participant_id`.
+
+1.  Each TSV file MUST contain a `session_id` column when
+  multiple [sessions](../glossary.md#session-entities)[^1] are present
+  in the data set regardless of whether those sessions are in
+  the `phenotype/` data, `sub-<label>/` data, or a combination of the two.
+
+1.  If more than one of the same measurement tool is acquired within
+  the same `session_id`, a `run` column MUST be added.
+
+1.  To encode the acquisition time for a measurement tool’s `session_id`,
+  add the `session_id` to the sessions file and
+  include the OPTIONAL `acq_time` column.
+
+To summarize this guideline as a table:
+
+<!-- This block generates a columns table.
+The definitions of these fields can be found in
+  src/schema/rules/tabular_data/*.yaml
+and a guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_columns_table("modality_agnostic.Phenotypes") }}
+
+Furthermore, if you have to add a `session_id` column to the
+tabular phenotypic data, you then MUST also introduce a session directory to the
+imaging data, even if only one imaging session has been created.
+This rule can be considered as "**if anyone uses sessions, everyone uses sessions**."
+And vice versa, if imaging data has session directories,
+all imaging data and tabular phenotypic data MUST have sessions.
+
+This produces a file in which same-participant entries can take up as many rows
+as needed according to the smallest unit of acquisition.
+The combination of values in the `participant_id`, `session_id`, and `run` (if present)
+columns MUST be unique for the entire tabular file.
+
+### 4. Add `MeasurementToolMetadata` to each tabular phenotypic measurement tool
+
+Whenever possible, it is RECOMMENDED to add `MeasurementToolMetadata` to
+each `phenotype/<measurement_tool_name>.json` data dictionary.
+This improves reusability and provides clarity about the measurement tool.
+
+### 5. Use the demographics file for common variables about participants
+
+Some studies collect demographics into their own tabular phenotypic data file already.
+In these cases, it is RECOMMENDED to house this data in the `phenotype/` directory
+as a TSV called `demographics.tsv` and its corresponding data dictionary JSON
+called `demographics.json`.
+
+### 6. Store longitudinal age in the demographics file
+
+It is RECOMMENDED to use the `age` column to record participant age
+at every session in longitudinal or multi-session data sets.
+This reduces data duplication across tabular data files. The `Units` of `age`
+do not have to be years so long as the units of the age
+are written in `phenotype/demographics.json`.
+Consider participant privacy or study objectives when selecting
+the `Units` of `age` or the accuracy of `age` data.
+
+### 7. Use the sessions file at the root level
+
+If there is more than one session for any one participant, then
+it is REQUIRED to provide a sessions file at the dataset root.
+The sessions file MUST list all sessions for all subjects across
+imaging and tabular phenotypic data.
+
+When a sessions file is in use, you MUST NOT provide additional sessions files
+at the participant-level which would otherwise use the inheritance principle.
+If a sessions file is provided, then it MUST begin with a `participant_id` column
+followed immediately by a `session_id` column. The data dictionary JSON file’s
+`session_id` field MUST include `Levels` with the description of each `session_id`.
+
+### 8. Record acquisition time of sessions with `acq_time`
+
+Whenever possible, it is RECOMMENDED to also collect acquisition time for
+tabular phenotypic data and store the time of acquisition[^2] of each row
+inside a column named `acq_time` in the sessions file.
+This is consistent with how acquisition time is recorded for MRI data
+and other time-sensitive measurements (for example systolic blood pressure).
+
+When needed to preserve participant privacy, you SHOULD record
+relative acquisition times with respect to the earliest session.
+Relative session acquisition times MAY be listed as durations from
+the earliest session (baseline) in days, months, or years
+using the `acq_time` column.
+
+## Summary
+
+This appendix described seven guidelines for best tabular phenotypic data.
+A short summary table here describes when to use which files.
+
+| File                           | Single session data | Multiple session data |
+| :----------------------------- | :------------------ | :-------------------- |
+| Participants                   | RECOMMENDED         | RECOMMENDED           |
+| Phenotypic and assessment data | RECOMMENDED         | RECOMMENDED           |
+| Sessions                       | OPTIONAL            | REQUIRED              |
+| Demographics                   | OPTIONAL            | RECOMMENDED           |
+
+## Examples
+
+What follows are a few common use case examples for tabular phenotypic files.
+
+### 1 participant session with both non-tabular and tabular phenotypic data
+
+File tree
+
+```Text
+phenotype/
+    <measurement_tool_name>.json
+    <measurement_tool_name>.tsv
+sub-01/anat/
+    sub-01_T1w.json
+    sub-01_T1w.nii.gz
+```
+
+Contents of `phenotype/<measurement_tool_name>.tsv`
+
+```tsv
+participant_id	measurement_1	measurement_2
+sub-01	value1	value2
+```
+
+### 1 participant with 2 sessions, where 1 session is only tabular phenotype and the other is only imaging
+
+With only one imaging and one phenotypic session each in this example you might want
+to merge both imaging and phenotypic data under one session. But it is more correct to
+have separate sessions for the imaging and phenotypic data, especially if
+the sessions were collected days, weeks, or months apart. You can denote both sessions
+and their acquisition time in the `sessions.tsv` file and have `session_id` `Levels` noted
+in the `sessions.json` sidecar. Below are a CORRECT and an INCORRECT example
+of prepared data following these guidelines.
+
+#### CORRECT
+
+File tree
+
+```Text
+phenotype/
+    <measurement_tool_name>.json
+    <measurement_tool_name>.tsv
+sub-01/ses-MRI/anat/
+    sub-01_ses-MRI_T1w.json
+    sub-01_ses-MRI_T1w.nii.gz
+```
+
+Contents of `phenotype/<measurement_tool_name>.tsv`
+
+```tsv
+participant_id	session_id	measurement_1	measurement_2
+sub-01	ses-pheno	value1	value2
+```
+
+#### INCORRECT
+
+File tree
+
+```Text
+phenotype/
+    <measurement_tool_name>.json
+    <measurement_tool_name>.tsv
+sub-01/anat/
+    sub-01_T1w.json
+    sub-01_T1w.nii.gz
+```
+
+Contents of `phenotype/<measurement_tool_name>.tsv`
+
+```tsv
+participant_id	measurement_1	measurement_2
+sub-01	value1	value2
+```
+
+A session directory **MUST** be present in the participant directory and
+the `session_id` column **MUST** be present in `<measurement_tool_name>.tsv` as well.
+Sessions must be used consistently for the combination of tabular and
+non-tabular phenotypic data.
+
+### 2 participants with a mix of tabular phenotypic data and imaging sessions
+
+File tree
+
+```Text
+phenotype/
+    <measurement_tool_name>.json
+    <measurement_tool_name>.tsv
+sub-01/
+    ses-MRI1/
+        anat/
+            sub-01_ses-MRI1_T1w.json
+            sub-01_ses-MRI1_T1w.nii.gz
+    ses-MRI2/
+        anat/
+            sub-01_ses-MRI2_T1w.json
+            sub-01_ses-MRI2_T1w.nii.gz
+sub-02/
+    ses-MRI1/
+        anat/
+            sub-02_ses-MRI1_T1w.json
+            sub-02_ses-MRI1_T1w.nii.gz
+```
+
+Contents of `phenotype/<measurement_tool_name>.tsv`
+
+```tsv
+participant_id	session_id	measurement_1	measurement_2
+sub-01	ses-pheno1	value1	value2
+sub-02	ses-pheno1	value3	value4
+sub-02	ses-pheno2	value5	value6
+```
+
+### 3 participants with 3 different kinds of sessions among them
+
+The `ses-baseline` session collects an MRI and tabular phenotypic data.
+
+File tree
+
+```Text
+participants.json
+participants.tsv
+sessions.json
+sessions.tsv
+phenotype/
+    demographics.json
+    demographics.tsv
+    ...
+sub-01/
+    ses-baseline/
+    ses-followupMRI/
+sub-02/
+    ses-baseline/
+sub-03/
+    ses-baseline/
+    ses-followupMRI/
+```
+
+Contents of `sessions.tsv`.
+
+```tsv
+participant_id	session_id	acq_time
+sub-01	ses-baseline	2001-01-01T12:05:00
+sub-01	ses-followupMRI	2001-07-01T13:33:00
+sub-01	ses-interview	2002-01-01T11:21:00
+sub-02	ses-baseline	2001-04-01T11:01:00
+sub-02	ses-interview	2002-04-01T14:08:00
+sub-03	ses-baseline	2001-09-01T11:45:00
+sub-03	ses-followupMRI	2002-03-01T12:17:00
+```
+
+Contents of `sessions.json`. Note how the `session_id` `Levels` are clearly described.
+
+```json
+{
+    "participant_id": {
+        "Description": "BIDS participant identifier"
+    },
+    "session_id": {
+        "Description": "BIDS session identifier",
+        "Levels": {
+            "ses-baseline": "Baseline visit for MRI and assessments",
+            "ses-followupMRI": "6-months after baseline MRI follow-up",
+            "ses-interview": "1-year after baseline in-person follow-up"
+        }
+    },
+    "acq_time": {
+        "Description": "When the data acquisition started"
+    }
+}
+```
+
+Contents of `participants.tsv`.
+
+```tsv
+participant_id	sex
+sub-01	M
+sub-02	F
+sub-03	F
+```
+
+Contents of `phenotype/demographics.tsv`. Measures or features that can change
+from session to session belong here especially.
+
+```tsv
+participant_id	session_id	age	gender	race	household_income
+sub-01	ses-baseline	10	3	4	5
+sub-01	ses-followupMRI	10	3	4	5
+sub-01	ses-interview	11	4	4	6
+sub-02	ses-baseline	9	1	3	3
+sub-02	ses-interview	10	1	7	3
+sub-03	ses-baseline	11	2	10	4
+sub-03	ses-followupMRI	12	5	10	4
+```
+
+For more complete examples, see the `pheno00*`
+[bids-examples on GitHub](https://github.com/bids-standard/bids-examples/).
+
+[^1]: A session is any logical grouping of imaging and behavioral data consistent
+across participants. Session can (but doesn't have to) be synonymous to a visit
+in a longitudinal study. In situations where different data types are obtained over
+several visits (for example fMRI on one day followed by DWI the day after)
+those can still be grouped in one session. Refer to the
+[definition of session](../glossary.md#session-entities) for more details.
+
+[^2]: Datetime format and the anonymization procedure are
+described in [Units](../common-principles.md#units).
diff --git a/src/common-principles.md b/src/common-principles.md
index eeb4170244..3b5a6dbd36 100644
--- a/src/common-principles.md
+++ b/src/common-principles.md
@@ -470,7 +470,7 @@ NIfTI header.
 
 ### Tabular files
 
-Tabular data MUST be saved as plain-text, tab-delimited values (TSV) files
+Tabular data MUST be saved as plain-text, tab-separated values (TSV) files
 (with [extension `.tsv`](glossary.md#tsv-extensions)),
 that is, [CSV files](https://en.wikipedia.org/wiki/Comma-separated_values) where commas are replaced by tab characters.
 Tabs MUST be true tab characters and MUST NOT be a series of space characters.
@@ -532,6 +532,12 @@ Note that if a field name included in the data dictionary matches a column name
 then that field MUST contain a description of the corresponding column,
 using an object containing the following fields:
 
+!!! success "Guideline 1"
+
+    For [best tabular phenotypic data](./appendices/phenotype.md):
+    Each tabular phenotypic data TSV file MUST be accompanied by
+    a corresponding data dictionary JSON file.
+
 <!-- This block generates a metadata table.
 The definitions of these fields can be found in
   src/schema/objects/metadata.yaml
diff --git a/src/modality-agnostic-files/data-summary-files.md b/src/modality-agnostic-files/data-summary-files.md
index d6b5032c2b..c2b8fe80b2 100644
--- a/src/modality-agnostic-files/data-summary-files.md
+++ b/src/modality-agnostic-files/data-summary-files.md
@@ -132,6 +132,43 @@ It is RECOMMENDED to accompany each `samples.tsv` file with a sidecar
 }
 ```
 
+## Demographics file
+
+Template:
+
+```Text
+phenotype/
+    demographics.tsv
+    demographics.json
+```
+
+The demographics file is an OPTIONAL tabular phenotypic file in
+the `phenotype/` directory meant to house common subject demographics.
+For example demographics like age, gender, race, and household income.
+A demographics file is RECOMMENDED to use when any participant has
+more than one session of any type.
+It does not replace the participants file, which is meant for unchanging data about
+each participant in the data set. It is instead a superset of the participants file,
+centralizing demographics across as many sessions as are available.
+
+!!! success "Guideline 5"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+    Some studies collect demographics into their own
+    tabular phenotypic data file already. In these cases, it is RECOMMENDED
+    to house this data also in the demographics file.
+
+!!! success "Guideline 6"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+    It is RECOMMENDED to use the `age` column to record participant age
+    at every session in longitudinal or multi-session data sets.
+    This reduces data duplication across tabular data files. The `Units` of `age`
+    do not have to be years so long as the units of the age
+    are written in `phenotype/demographics.json`.
+    Consider participant privacy or study objectives when selecting
+    the `Units` of `age` or the accuracy of `age` data.
+
 ## Scans file
 
 Template:
@@ -199,9 +236,10 @@ meg/sub-control01_task-rest_split-02_meg.nii.gz	1877-06-15T12:15:27
 
 ## Sessions file
 
-Template:
+Template A (segregated sessions files):
 
 ```Text
+[sessions.json]
 sub-<label>/
     sub-<label>_sessions.tsv
 ```
@@ -209,11 +247,11 @@ sub-<label>/
 Optional: Yes
 
 In case of multiple sessions there is an option of adding additional
-`sessions.tsv` files describing variables changing between sessions.
+`sessions.tsv` files describing each session and variables changing between sessions.
 In such case one file per participant SHOULD be added.
 These files MUST include a `session_id` column and describe each session by one and only one row.
 Column names in `sessions.tsv` files MUST be different from group level participant key column names in the
-[`participants.tsv` file](./data-summary-files.md#participants-file).
+[`participants.tsv` file](#participants-file).
 
 <!-- This block generates a columns table.
 The definitions of these fields can be found in
@@ -223,7 +261,7 @@ and a guide for using macros can be found at
 -->
 {{ MACROS___make_columns_table("modality_agnostic.Sessions") }}
 
-`_sessions.tsv` example:
+`sub-<label>/sub-<label>_sessions.tsv` example:
 
 ```tsv
 session_id	acq_time	systolic_blood_pressure
@@ -231,3 +269,63 @@ ses-predrug	2009-06-15T13:45:30	120
 ses-postdrug	2009-06-16T13:45:30	100
 ses-followup	2009-06-17T13:45:30	110
 ```
+
+Template B (aggregated sessions file):
+
+```Text
+sessions.tsv
+sessions.json
+```
+
+Optional: Yes
+
+An aggregated sessions file CAN be provided at the dataset root.
+If a root-level sessions file is provided, then it MUST begin with
+a `participant_id` column followed immediately after by a `session_id` column.
+The intent of this root-level sessions file is to describe the sessions
+in a data set and non-demographic variables changing between sessions.
+Participant's demographic variables should be added to
+a [demographics file](#demographics-file), as described above.
+
+`sessions.tsv` example:
+
+```tsv
+participant_id	session_id	acq_time	systolic_blood_pressure
+sub-01	ses-predrug	2009-06-15T13:45:30	120
+sub-01	ses-postdrug	2009-06-16T13:45:30	100
+sub-01	ses-followup	2009-06-17T13:45:30	110
+sub-02	ses-predrug	2009-06-22T12:22:05	105
+sub-02	ses-postdrug	2009-06-23T12:22:05	95
+sub-03	ses-postdrug	2009-06-30T14:06:40	115
+sub-03	ses-followup	2009-07-01T14:06:40	120
+```
+
+!!! success "Guideline 7"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+    If there is more than one session for any one participant, then it is
+    REQUIRED to provide a sessions file at the dataset root.
+    The sessions file MUST list all sessions for all subjects
+    across imaging and tabular phenotypic data.
+
+    When a sessions file is in use, you MUST NOT provide additional sessions
+    files at the participant-level which would otherwise use
+    the inheritance principle. If a sessions file is provided, then
+    it MUST begin with a `participant_id` column followed immediately by
+    a `session_id` column. The data dictionary JSON file's `session_id` field
+    MUST include `Levels` with the description of each `session_id`.
+
+!!! success "Guideline 8"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+    Whenever possible, it is RECOMMENDED to also collect acquisition time
+    for tabular phenotypic data and store the time of acquisition of each row
+    inside a column named `acq_time` in the sessions file.
+    This is consistent with how acquisition time is recorded for MRI data
+    and other time-sensitive measurements (for example systolic blood pressure).
+
+    When it is needed to preserve participant privacy, you SHOULD record
+    relative acquisition times with respect to the earliest session.
+    Relative session acquisition times MAY be listed as durations from
+    the earliest session (baseline) in days, months, or years
+    using the `acq_time` column.
diff --git a/src/modality-agnostic-files/phenotypic-and-assessment-data.md b/src/modality-agnostic-files/phenotypic-and-assessment-data.md
index d876a38ee7..de3bf608db 100644
--- a/src/modality-agnostic-files/phenotypic-and-assessment-data.md
+++ b/src/modality-agnostic-files/phenotypic-and-assessment-data.md
@@ -18,16 +18,62 @@ Each of the measurement files MUST be kept in a `/phenotype` directory placed
 at the root of the BIDS dataset and MUST end with the `.tsv` extension.
 Filenames SHOULD be chosen to reflect the contents of the file.
 For example, the "Adult ADHD Clinical Diagnostic Scale" could be saved in a file
-called `/phenotype/acds_adult.tsv`.
+called `phenotype/acds_adult.tsv`.
 
 The files can include an arbitrary set of columns, but one of them MUST be
 `participant_id` and the entries of that column MUST correspond to the subjects
 in the BIDS dataset and `participants.tsv` file.
 
+!!! success "Guideline 2"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+    It is REQUIRED to aggregate all participant data into
+    one TSV per tabular phenotypic file.
+
+In phenotypic and assessment data each measurement tool has
+an independent aggregated data TSV file in which the user collects
+all subjects, sessions, and/or runs of data as one entry per row
+(with a row defined by the smallest unit of acquisition). In other words:
+
+1.  Each row MUST start with `participant_id`.
+
+1.  Each TSV file SHOULD contain a `session_id` column
+when multiple [sessions](../glossary.md#session-entities) are present
+in the data set regardless of whether those sessions are in
+the `phenotype/` data, `sub-<label>/` data, or a combination of the two.
+
+1.  If more than one of the same measurement tool is acquired
+within the same `session_id`, a `run` column SHOULD be added.
+
+1.  To encode the acquisition time for a measurement tool’s `session_id`,
+add the `session_id` to the sessions file
+and include the OPTIONAL `acq_time` column.
+
+!!! success "Guideline 3"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+
+    | **Column name**  | **Requirement** | **Description** |
+    | :--------------- | :-------------- | :-------------- |
+    | `participant_id` | REQUIRED        | MUST be the first column in the file. Note that data for one participant MAY be represented across multiple rows in case of multiple sessions or runs, and therefore the entry in the `participant_id` column will be repeated. |
+    | `session _id`    | CONDITIONAL ; If sessions are defined in the dataset | A `session_id` column MUST be added to all tabular files in the phenotype directory as soon as multiple sessions are present in the data set regardless of whether those sessions are in the  `phenotype/` data, `sub-<label>/` data, or a combination of the two. |
+    | `run`            | CONDITIONAL ; If there are multiple runs within any session | A chronological `run` number is used when a measurement tool or assessment described by a tabular file was repeated within a session. |
+    | `acq_time`       | OPTIONAL        | If acquisition time is available, the `acq_time` column CAN be used to record the time of acquisition of each row in the tabular file. |
+
+    Furthermore, if you have to add a `session_id` column to the tabular phenotypic data, you then MUST also introduce a session directory to the imaging data, even if only one imaging session has been created. This rule can be considered as "**if anyone uses sessions, everyone uses sessions**." And vice versa, if imaging data has session directories, all imaging data and tabular phenotypic data MUST have sessions.
+
+    This produces a file in which same-participant entries can take up as many rows as needed according to the smallest unit of acquisition. The combination of values in the `participant_id`, `session_id`, and `run` (if present) columns MUST be unique for the entire tabular file.
+
 As with all other tabular data, the additional phenotypic information files
 MAY be accompanied by a JSON file describing the columns in detail
 (see [Tabular files](../common-principles.md#tabular-files)).
 
+!!! success "Guideline 1"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+    Each tabular phenotypic data TSV file MUST be accompanied by
+    a corresponding data dictionary JSON file.
+
 In addition to the column descriptions, the JSON file MAY contain the following fields:
 
 <!-- This block generates a metadata table.
@@ -73,6 +119,13 @@ Please note that in this example `MeasurementToolMetadata` includes information
 about the questionnaire and `adhd_b` and `adhd_c_dx` correspond to individual
 columns.
 
+!!! success "Guideline 4"
+
+    For [best tabular phenotypic data](../appendices/phenotype.md):
+    Whenever possible, it is RECOMMENDED to add `MeasurementToolMetadata` to
+    each `phenotype/<measurement_tool_name>.json` data dictionary.
+    This improves reusability and provides clarity about the measurement tool.
+
 In addition to the keys available to describe columns in all tabular files
 (`LongName`, `Description`, `Levels`, `Units`, and `TermURL`) the
 `participants.json` file as well as phenotypic files can also include column
diff --git a/src/schema/objects/files.yaml b/src/schema/objects/files.yaml
index b16a8c9c1d..5d9b77a891 100644
--- a/src/schema/objects/files.yaml
+++ b/src/schema/objects/files.yaml
@@ -68,12 +68,15 @@ participants:
   display_name: Participant Information
   file_type: regular
   description: |
-    The purpose of this RECOMMENDED file is to describe properties of participants
-    such as age, sex, handedness, species and strain.
+    The purpose of this RECOMMENDED file is to describe unchanging properties of participants
+    such as sex, species, and strain.
     If this file exists, it MUST contain the column `participant_id`,
     which MUST consist of `sub-<label>` values identifying one row for each participant,
     followed by a list of optional columns describing participants.
     Each participant MUST be described by one and only one row.
+    For participants with multiple sessions, see
+    the [sessions file](SPEC_ROOT/modality-agnostic-files/data-summary-files.md#sessions-file)
+    and [demographics file](SPEC_ROOT/modality-agnostic-files/data-summary-files.md#demographics-file) sections.
 
     The `participant_id` entries MUST be a superset of all subject directories
     and all `participant_id` entries found among phenotypic and assessment data
diff --git a/src/schema/rules/tabular_data/modality_agnostic.yaml b/src/schema/rules/tabular_data/modality_agnostic.yaml
index fcaa4b32d4..12378ea271 100644
--- a/src/schema/rules/tabular_data/modality_agnostic.yaml
+++ b/src/schema/rules/tabular_data/modality_agnostic.yaml
@@ -18,6 +18,42 @@ Participants:
   index_columns: [participant_id]
   additional_columns: allowed
 
+Phenotypes:
+  selectors:
+    - extension == ".tsv"
+  initial_columns:
+    - participant_id
+  columns:
+    participant_id:
+      level: required
+      description_addendum: |
+        MUST be the first column in the file.
+        Note that data for one participant MAY be represented across multiple rows
+        in case of multiple sessions or runs, and
+        therefore the entry in the `participant_id` column will be repeated.
+    session_id:
+      level: optional
+      description_addendum: |
+        REQUIRED if sessions are defined in the dataset.
+        A `session_id` column MUST be added to all tabular files in the phenotype directory
+        as soon as multiple sessions are present in the data set
+        regardless of whether those sessions are in the
+        `phenotype/` data, `sub-<label>/` data, or a combination of the two.
+    run:
+      level: optional
+      description_addendum: |
+        REQUIRED if there are multiple runs within any session.
+        A chronological `run` number is used when
+        a measurement tool or assessment described by a tabular file
+        was repeated within a session.
+    acq_time:
+      level: optional
+      description_addendum: |
+        If acquisition time is available, the `acq_time` column CAN be used
+        to record the time of acquisition of each row in the tabular file.
+  index_columns: [participant_id, session_id, run, acq_time]
+  additional_columns: allowed
+
 Samples:
   selectors:
     - path == "/samples.tsv"