The flight number has basically two parts, the first two characters identify the airline selling the flight and the remaining digits are the airline's flight identifier. The first two digits are the IATA assigned airline code, which were originally based on the airlines name, ie: BA - British Airways, DL - Delta, etc.

A flight is uniquely identified in the global aviation system through a combination of three distinct codes: the Flight Number, the Callsign, and the Aircraft Tail Number. The Flight Number (e.g., DL123) is a commercial identifier used by passengers and airlines for scheduling. The Callsign (e.g., "Delta 1-2-3") is what Air Traffic Control uses to communicate with the pilot in real-time; while often similar to the flight number, it may include letters to prevent confusion with similar-sounding flights. However, the only truly unique "serial number" for the physical vehicle is the Tail Number (or Registration), such as N123AA. In 2026, many flight tracking apps also use a 24-bit ICAO Hex Code, which is a unique digital address broadcast by the aircraft's transponder, ensuring that even if two planes have similar callsigns, the digital system never confuses their specific hardware or location.