purpose of \kern-\alignsep@ in \math@cr@@@align

[Udi-Fogiel]

is it just to compensate in the case of a tag (equation number)?

[Udi-Fogiel]

The reason I'm asking is because when the page direction is RTL then this kern ends up shifting the whole line (maybe it is considered at the start and not at the end of the line). Babel currently use this following hack:
https://github.com/latex3/babel/blob/67ef7a285ef94be9116c28b9af142a7a59af3ee9/babel.dtx#L16672-L16674
I'm wondering if \kern-\alignsep@ can be placed inside the \iftag@ branch of \math@cr@@@align instead.

[Udi-Fogiel]

The reason I'm asking is because when the page direction is RTL then this kern ends up shifting the whole line (maybe it is considered at the start and not at the end of the line).

No, the alignment is LTR (inherits \textdirection which is LTR when the equation has started), so it is at the end of the line, but inside RTL vbox the boxes are stacked vertically aligned to the right edge of the box.