http://hl7.org/fhir/StructureDefinition/iso21090-ADXP-additionalLocator|4.0.1

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).