Thanks for this! I would like to take it one step further and add a Units dropdown on the panel itself.

Instead of going into Prefs or add Rulers to change units, add a dropdown to the area.
I constantly have to change units based on job parameters.

THX

Yes, Lewis, I do agree with the comment. This is bonkers and I hope got have it at least clarified in the app. Thanks for raising this issue, because I just had no time to log it myself! Hope there would be others to upvote this.

Thanks for clarifying. I don't see the point of having the global change of units in preferences. It just creates confusion and does not work as expected. Essentially, one should just ignore the preference setting and use the document setup to change units.

Another area of confusion with units is the Properties palette. Selecting a different unit here only affect the local document, whilst changing other settings (Ruler & Grids, Guides, Use Preview Bounds, Scale Corners, Scale Strokes & Effects) are global changes.

Also, if I chose another unit of measurement in Preferences, the Properties palette updates to show that unit of measurement. But if I chose a different unit of measurement in the Properties palette, the units don’t change in Preferences. This doesn’t work as expected.

Treat those in Document Setup as 'local' units. You can also choose them by clicking with the right mouse button at the intersection of rulers.
These are responsible for all ruler-nased sizes in a document — dimensions, coordinates, grids... They are more important.

Those defined in Preferences are 'global' units. These force the units across all documents once changed. Try to create a Print-based doc, and a Web-based doc (they use 'mm' (if you are not i US) and 'px').

If you change 'global' general units in Preferences to 'mm', you will get your rulers switched to local 'mm' in all opened documents.

If you change them again to 'px' — all your docs will get 'px' as local units.

But changing these only in Document Setup (locally) won’t change other opened documents.

This is a fairly obscured behaviour, I agree! But once you learn it, it makes sense. Ai just never cared to explain it properly (and I learn it by chance myself).

Does it make sense for you?

Egor, units in Preferences is millimeters and in Document Setup it's pixels.

Lewis, can you also check two places:
Units in Preferences and Document Setup?
What do they say?