Post Exchange

An exchange is a type of retail store found on United States military installations worldwide. Originally akin to trading posts, they now resemble contemporary department stores or strip malls.

Exchanges are commonly called a Base Exchange (BX) on U.S. Air Force installations, and they are referred to as a Post Exchange (PX) in the U.S. Army. Exchanges on Army and Air Force installations are run by the Army and Air Force Exchange Service (AAFES). Every exchange operated by the AAFES has been rebranded simply as an Exchange to simplify differentiation between the similar-sounding BX and PX names.

U.S. Navy installations and ships use a Navy Exchange (NEX) whereas the U.S. Marine Corps use a Marine Corps Exchange (MCX) for naval and marine locations afloat and ashore.

Base Exchanges sell consumer goods and services to authorized patrons such as Active Duty, Reserve, National Guard, Retired Members of the U.S. Uniformed Services and their dependents. Other authorized patrons include Honorably Discharged Veterans certified 100% disabled and/or Totally and Permanently Disabled (TPD) by the Veterans Administration, Recipients of the Medal of Honor, Military Transition Personnel, DoD Employees when stationed outside the United States, Red Cross Personnel who are U.S. citizens assigned outside the United States with the Military Service. Authority to use these facilities is normally determined by presentation of the U.S. Uniformed Services Privilege and Identification Card, either when entering the store or when paying for goods and services.

...
Wikipedia