addr:BuildingType_type

This is a modifier to the BuildingId element and this
cannot exist without it. The type of structure (when
several structures are found at the same address), e.g.,
Apartment, Tower, Block. Used with Building Identifier
to designate one of several structures at a given site.
Fits within the general USPS definition of a "secondary
address designator" and EPA definitions of "secondary
address identifier"