Based on what I understand, you use level:ref=*
I could certainly be wrong, but I think this is how it works. For example, let’s take a building that has six levels, but does not have a floor number “4”. Level 0 would be the ground floor. The rest of the floors would have the following tags:
level=1
level:ref=1
level=2
level:ref=2
level=3
level:ref=3
level=4
level:ref=5
level=5
level:ref=6
level=6
level:ref=7