Mail to US Military overseas addresses and diplomatic pouch (APO = Army Post Office, FPO = Fleet Post Office, DPO = Diplomatic Post Office) is a little weird:
The address line(s) are dependent on the destination. Land-based units will need the mail center/box number. Navy (FPO) is different: mail destined to a sailor aboard ship should have the ship's name and number as the address, something like
Seaman Sarah Cambell USS Enterprise (CVG-65) FPO AP 96543-1234
The city field is one of
- APO (Army Post Office)
- FPO (Fleet Post Office)
- DPO (Diplomatic Post Office)
- The state field is one of
- AA (Armed Forces Americas, serving all of the Americas except Canada, including military bases within the United States).
- AE (Armed Forces Europe, serving Europe, Canada, Africa and the Middle East).
- AP (Armed Forces Pacific, serving Asia and the Pacific).
- The zip code is the military zip code, tied to the gateway designated by the state field.
More at https://stamps.custhelp.com/app/answers/detail/a_id/774