addr:FloorId_type

The number, letter, or word or combination of numbers
and letters distinguishing one floor from another within
a structure.
Used with Floor Type to designate which floor of a given
structure contains the address. If only a Floor name
appears (such as Mezzanine) it should be parsed as a
Floor Identifier rather than as a Floor Type. Fits
within the general USPS definition of a "sescondary
address designator" and the EPA definitions of
"secondary address identifier."