Airline designator codes or reservation codes follow the format xx (a), i.e., two alphanumeric characters (letters or digits) followed by an optional letter. These designators are used to identify an airline for commercial purposes in reservations, timetables, tickets, tariffs, air waybills and in telecommunications.
There is also an accounting or prefix code which is used on tickets as the first three characters of the ticket number.
International Civil Aviation Organization (ICAO) also has assigned a code for the aircraft operating agencies, aeronautical authorities, and services related to international aviation, each of whom is allocated both a three-letter designator and a telephony designator.
Most airlines employ a call sign that is normally spoken during airband radio transmissions. As by ICAO Annex 10 chapter 184.108.40.206.2.1 a call sign shall be one of the following types:
- Type A : The characters corresponding to the registration marking of the aircraft.
- Type B : The telephony designator of the aircraft operating agency, followed by the last four characters of the registration marking of the aircraft.
- Type C : The telephony designator of the aircraft operating agency, followed by the flight identification.
The one most widely used within commercial aviation is type C. The flight identification is very often the same as the flight number, though this is not always the case. In case of call sign confusion different flight identification can be chosen, but the flight number will remain the same. Call sign confusion happens when two or more flights with similar flight numbers fly close to each other.