The history of orthodontics has been intimately linked with the history of dentistry for more than 2000 years. Dentistry had its origins as a part of medicine.
Addressing an envelope to a post office box address is just like addressing envelopes to any other type of address. Of course, before you can address the envelope, you need the name of the recipient, the post office box number and the city, state and ZIP code.
While the layout of each APO/FPO/DPO varies by location, suffice it to say they look and operate just like a real Post Office. In fact, they have to, since we are talking about U.S. Mail. Each APO/FPO/DPO address holder has a mail lock box in which their mail is slotted. Since parcels can't fit in these boxes, delivery or notification slips, just like real ones, are put in the boxes whenever a parcel arrives.