Having sat in the back of an AA MD
-80, I think I know.
If the seating is 2-3, then the rear galley is actually 2 narrow galleys, one aft of the other. If you look, you will see one galley that has the service door. Then, there is a wall the separates the second galley. It has one window.
If the seating is 3-2, then one galley, that is about the same width as those used on Boeing narrowbodies, is installed. This leaves room for two rows of seats on the port side between the galley and the port lav.
Since 2-3 has been the traditional arrangement for Douglas and Fokker aircraft, this is probably the standard seating arrangement, while 3-2 is optional.
Theoretically, the service door can't be on the starboard side, because the aft cargo hatch is on the starboard side.