https://en.wikipedia.org/wiki/Tail_Code
U.S. Air Force. Since 1993, the U.S. Air Force (USAF), Air Force Reserve, and Air National Guard (ANG), except Air Mobility Command (AMC) aircraft, utilize this format in all tail codes: two large letters, followed by two digits printed in a smaller text size, followed by three more digits printed in a larger text size.. The first two letters identify the home base, or in some organizations, a ...