Definition: Flight number (aircraft)
A flight number is assigned by the air transport operator to an aircraft for identification purposes for use by air traffic control. It consists of two elements: (1) a three letter code identifying the operator and (2) a four digit flight number.
Passengers using a flight by an aircraft may be travelling under a range of different flight numbers. Only the active flight number for an aircraft used for air traffic control purposes is in question here. The four-digit flight number can be extended with a further alphabetic character (A, B etc.) to cover the case of duplicated flights.
Draft Glossary on Air Transport Statistics, 2001