Just a flag that makes monsters invisible, non-solid and non shootbale if they are deactivated. Maybe it could be like they have NOINTERACTION when not active, plus INVISIBLE.
It would help to put monsters on maps, with all the settings of arguments, mapeditors flags and stuff, but just use they later on the map. For example, player puts a monster with certain values on its arguments, plus "deaf" and stuff, but just want it to appear when player is revisiting that sector. if he tries to spawn that monster, he would have to set it a TID, and use ACS to set it's arguments and all other stuff. This way, he could just activate the monster.