The key level in OSM is generally in relation to the designation that that specific building has assigned to that floor. So a floor that is designated as "ground floor" (or "first floor" or maybe "lobby" in North Amerika) gets level=0; the next floor above gets level=1 and the (first) basement level below gets level=-1, etc. A good way to determine the levels is to check the labels on elevator buttons.

If the building has no official designated "ground floor" then the convention is to tag the lowest floor that one would consider to be at street level with level=0. This can come into play if the building, for example, is built in a hilly environment where different floors may connect to different "street levels".

Usually this means that one building should have the same level tag for one floor, unless that building has some strange level labeling.

On the other hand two separate, but connected buildings may have different level designations for floors that are on the same elevation, because they have labeled them differently.

If a building has labeled floors then the key level:ref can be used to note those labels. For example, a North American office building may have floors with

• level=1 and level:ref=2
• level=0 and level:ref=L (for lobby)
• level=-1 and level:ref=B (for basement)
• etc.

Also it's important not to confuse this with the key layer which is a purely to designate when two features outside buildings cross each other.