You have activities which will certainly end up in an unspecified room

[Liviu Lalescu]

No, FET will try again and will continue to generate the timetable: displace the "normal" activities from the Labs and put the "special" activities in the Labs. You might want to try practically.

Wrong. You misunderstood the problem. I tried practically and your idea doesn't work (only in special case, but not in general)

of course you can do it you way, but ONLY if you do different constraints for normal AND special rooms.

the suggestion was to simplify the problem by one of this tricks:
add a single constraint for all activities AND special room.

But that is currently impossible, because there is no "home" room constraint. So you will end up in one of this problems:
a) it is impossible to generate at all (if "all" activities got only "normal" rooms.
b) "normal" rooms might end up in special rooms (if "all" activities" got "all rooms")

I see.

[Volker Dirr]

But at the end it doesn't matter, since i am not sure if that "activities home" constraint will be fine. It will solve the problems i wrote, but the results will still "messed", because in normal case activities are split; and they will be split into different rooms. But caring about that will be slow down too much (i bet).

So the suggestion will be only fine if you schedule workshops. Maybe also for examination timetables at universities. But the "normal" schools and university timetables shouldn't use such a feature. I is too much mess. The tables will be bad.