GLOSSARY
Secondary address unit designator
By Justin Winter · Updated August 28, 2026
The abbreviation USPS uses for a unit within a building — Apt, Ste, Unit, Fl — placed after the street address to route mail within a multi-unit property.
USPS publishes a standard list of unit designators (Apartment, Suite, Unit, Floor, and others) and their approved abbreviations in its addressing standards, so that a piece for one unit in a building is not delivered to another.
Leaving off a required unit designator, or using a non-standard one, is a common cause of an address failing standardization even when the street address itself is correct.
See also
- Address standardization — Rewriting an address into the exact form USPS uses — canonical abbreviations, directionals, unit designators, and ZIP+4.
- Delivery Point Validation — A USPS check that confirms an address is a real, individually deliverable delivery point — down to the apartment or suite.
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