This can be a unit designator, such as apartment number, suite number, or floor. There may be several unit designators in an address (e.g., "3rd floor, Appt. 342".) This can also be a designator pointing away from the location (e.g. Across the street from).
\nhttp://terminology.hl7.org/CodeSystem/v3-AddressPartType
where concept is-a ADL