Issue By Armed Service Location Designator 

Type - Data Synonym
Description: APO (Army post Office) / FPO (Fleet Post Office) represents the Military Postal Center code for military postal locations outside the continental United States (OCONUS) corresponding to the contract's issuer. Army and Air Force locations use APO, and Navy, USMC, and Coast Guard use FPO designations. (DoD 4525.6-M). This field will also contain the MSPA pseudo-state code of AA, AE, or AP. 
Information Exchange: Payment Request