"2 or 3 character code as designated by the International Civil Aviation Organisation (ICAO) to uniquely designate an airline operator. Local (non-ICAO) codes can be added as required as long as they are unique for airline operators within the defined context.
ICAO defined code for the company owning the aircraft. Ex: SAS, NAX. Two or three characters. IATA and ICAO codes can be identical. The ICAO standard uses 3 character codes, but in cases where no ICAO standard code exists, an alternative code can be used. This can often be the 2 character IATA code if one exists.
Reference Document: ICAO document 8585"
