How can I make sure that only low and medium security inmates are in a specific area? Is it possible to reverse the overlay you can set? It seems like you are only able to clear areas for low OR medium security prisoners, but sharing is not possible. The thing I want here is to make sure that max security prisoners are not in certain areas.
Yes, you can, but it will be tricky:
'Wall-in' the area that you'd like restricted for MaxSecs. Then create several hallway-like entrances to that area depending on the number of security-categories that you'd like allowed.
Now, restrict each hallway to respective allowed category and make the walled-in area shared.
Though in this setup the area is technically shared, but MaxSec prisoners will have no way of accessing it as each of possible entrances is restricted for them.
Update: I've done some testing and it indeed works as expected:
I had only 2 MinSec and 6 MedSec prisoners (typical start game). 1 of each got interested in Kitchen-work. Only one got signed up, as the other has no possible route to get there. In both holding cells only prisoners of certain type are found. They do not go 'at will' through restricted areas. Also, should a case of no route exist and a prisoner needed to go somewhere - he gets escorted by guards. E.g. when I zoned top holding cell and hallway entrance to it to MaxSec, there was no way for them to exit the area, so they all were escorted out by guards through now restricted to them hallway.
No, if you set an area as shared it's for all classes of prisoner. If you want a room/section for more than one type, but not all, you have to build it for each prisoner type and assign only that type via deployment.
Having said that, you can achieve something close using different times in the regime for different groups to have access to the same room. e.g. sharing the canteen.