Military mail is post addressed through a forces or diplomatic postal service rather than to a place: in the United States an APO (Army or Air Force Post Office), FPO (Fleet Post Office) or DPO (Diplomatic Post Office), written with a unit, PSC or CMR number and the codes AA, AE or AP where a state would go; for the British forces a BFPO number; for the Canadian forces a CFPO number.
What Spaw answers
address_type reads military in the United States, the United Kingdom and Canada, the three countries whose address rules carry these words. APO, FPO, DPO, BFPO and CFPO are read anywhere in a line; PSC and CMR only at the start of one and followed by a number. The Armed Forces codes AA, AE and AP are read as the state, like any other US state code.
It carries no risk weight, and it is not a box. The routing line stands in for the street, so missing_street does not fire, and the unit or PSC line stays in street. The British postcode pattern accepts a BFPO number where the postcode goes. Great Britain's postcode directory lists neither BFPO numbers nor the BF1 codes, so it is not asked about them: a BFPO number is accepted and answers postcode_status null — not evaluated — and never postcode_not_found. exists stays null: none of the three countries has a complete register of buildings, and none of their registers lists a forces post office.
What it is not
It is not a place. The address tells the forces' own postal service where to route the post, and nothing in it says where anybody is — or who they are.