The table includes those facilities listed as post offices in the annual reports. It does not include other post office type facilitites, such as classified branches and classified stations. Here are the USPS”s definitions of a few of these facilities:
main post office (MPO) – The primary postal facility in a city where the post office uses subordinate post office stations or post office branches. It provides complete postal services to a specified geographic area. (Formerly called general post office.)
post office branch – A unit of a main post office that is outside the corporate limits of the city or town of the main post office. (Compare with post office station.) (Also called classified branch.)
post office station – A unit of a main post office that is within the corporate limits of the city or town of the main post office. (Compare with post office branch.)
Source: U.S. Postal Service, Publication 32: Glossary of Postal Terms (Washington: U.S. Postal Service, July 5, 2007).