The callbacks folder is automatically created at the top level of the nodeStorage folder when the first post is published.
It has a single sub-folder called publish. Each script in that folder will be called for each post that's published. A script is a file with a .js extension. The script has four values set up as globals: relpath, body, type and screenName.
relpath is the path to the file that's being written relative to the base of the user's folder. Something like 2018/05/02/0012.html.
body is the contents of the file.
type is a mime type, something like text/html or application/json.
screenName is the name of the user who is publishing the file.
It's enough information to mirror the file in another location, as an example. I use it to keep a copy of my writing in a folder on scripting.com.
Here's an example of a callback script for publish.