Just Post

I've seen B and SB (basement and subbasement) on elevator buttons. Generally those are floors that the public isn't allowed to go to and I never had the right key to activate them so I don't know what was there.

I've also seen B1 B2 B3 B4 B5 for different basement levels, though I don't remember which was on top. Those were all parking levels though so no rooms were numbered.

Are you in the US, and what kind of building is it?

I don't ever remember seeing a separate ground and 1st floor in a building, although I haven't been in every building.

Usually I’ve seen rooms starting with zero as a basement level, although due to terrain (such as being on a hillside) there could still be direct access to the outside ground. This is especially the case in residential/apartment buildings where windows and direct emergency egress is legally required. Things get wacky when the terrain around a building is not uniformly flat. It can be even weirder in big cities like New York where there might be an entrance at the street level but the hotel reception is up a level because the entrance to a parking garage is at the street level, then above the lobby level there’s maybe a mezzanine level with conference rooms, and then above that is the floor where rooms start and while an American might call that floor 4 or a European would call it floor 3, it might be labeled in the elevator as floor 1, 2, 3, or 4.

The things I have always seen as consistent in the US is whatever number a hotel room starts with is the number you press in the elevator. If you’re in room 647 press floor 6; if you’re in room 1232 press floor 12. Also, whatever level has a star ⭐️ in the elevator is the floor they consider the level with the main entrance. You’ll find the reception/front desk there and it should be obvious where to go for a taxi. It might not be the level to go out if you’ve parked a car, though, especially if the hotel has a parking garage.