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, rather than specifying a smaller location within some larger one (e.g., Dutch "t.o." means "opposite to" for house boats located across the street facing houses).

unique id for the element within a resource (for internal references).

An Extension

Source of the definition for the extension code - a logical name or a URL.

The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition should be version specific. This will ideally be the URI for the Resource Profile defining the extension, with the code for the extension after a #.

Value of extension - may be a resource or one of a constrained set of the data types (see Extensibility in the spec for list).