APO addresses and boxes belong to the United States military. APO stands for Army Post Office and is normally associated with Air Force or Army installations. The person to whom the box is assigned is not public information, so he must voluntarily choose to reveal his identity. All APO boxes can only be sent out or picked up via the United States Postal Service.
All APOs are assigned a unique zip code. If this zip code is known, tracking the location of the military installation is easy. An embassy abroad may also use an APO or FPO address If he hasn't changed his address to a DPO. DPO stands for Diplomatic Post Office and represents US diplomatic missions overseas.
Bear in mind that there are special customs requirements for sending items to an APO address. Restrictions vary by APO address, so it is advisable to contact the local post office for further information before sending. Some common restrictions are large and heavy items exceeding 70 pounds and 130 inches in combined length and girth, pressurized items such as shaving cream, firearms, alcohol, perishable foodstuffs and other hazardous materials. For specific questions regarding these restrictions, contact the Military Post Office Service.