So apartments somehow have both lot zoning 8 (base) and 9 (sub). When you are on the lot, it is only ever 9 as far as I can see. In what cases is it 8? I would like not to check for 8 if it never happens.

When you move a sim in to an apartment, it makes a sublot copy of the baselot. That way, you can edit the sublot (your sim can redecorate, change doors, change furniture etc.,) without affecting the baselot at all. When the sim is moved out of the apartment, that sublot is removed.

So it seems that you can't ever walk on the base lot 8 and interact with objects without being on your sublot 9, and objects would see zone 8 only while it is being built (like lamps). What happens to objects in the shared space when an apartment is removed? While you cannot legitimately build there, something might get dropped programmatically, gift, empty containers, weeds and such. Is that also reset?

The objects of an Aparment lot are stored in memory, presumably to facilitate lot restoration for other households. How are these object's marked? They must be marked in some way because the Fridge knows not to display them among leftovers. They don't seem to have any special Flags. I have resigned from doing anything with house inventory on apartment zoning for now.