This is the room number you are currently editing. 12 stands for the zone you are in, and 99 stands for the specific room in the zone that you are editing.
Unlike all other types of files, rooms use decimals so that we may increase the number available per zone. Example - objects for zone 12 are #1200 - #1299. Rooms for zone 12 are 12.0 - 12.3200.
Because zedit is also associated with a room number zedit also uses the decimal based numbering system.