I can't tell you exactly - because, first, I don't remember, but besides that what if that unit is occupied when you check in? - but I do know I was able to be transferred to Special Services before arrival; and I also know things change.
Anyway, I originally just specified the need for a first floor unit or elevator building because, well, they haven't perfected the stair-climbing
ecv 
. But when I got to my room, I realized I wouldn't be able to easily get into the bed. The next day, I moved to an HP unit in Building 24 or 25, next-to-last to the right when you face the building.