FPO is a short form used in American military addresses. It stands for Field Post Office or Fleet Post Office. When someone in the US army or navy is serving away from an ordinary town, their letters often do not use a normal city name in the same way; the mail goes through this military post office, and you write FPO as part of the address, the way you might write a city. Fleet Post Office is the navy side, for ships and the sea; Field Post Office is the land side. Ordinary people in Taiwan almost never need this, except when writing to someone in the US forces. Though actually, you should not invent the rest of a military address; the unit’s own mail instructions matter more than guessing from the three letters.

美軍軍郵（戰地郵局或艦隊郵局的縮寫，寫在軍人地址裡）