Similar to the International Air Transportation Association (IATA) codes, the ICAO airline designator codes are assigned by the International Civil Aviation Organization (ICAO) to aircraft operating agencies, aeronautical authorities, and services. Unlike IATA codes, the ICAO codes are unique for each airline and do not have limits on the number of codes that can be issued.
All aircraft operating agencies or airlines, aeronautical authorities, and services related to international aviation are allocated both a three-letter designator and a telephony designator. All the designators are listed in ICAO Document 8585: Designators for Aircraft Operating Agencies, Aeronautical Authorities, and Services. For example, a three-letter designator would be ‘AAL’, while a telephony designator would read 'AMERICAN', and the operator would be "American Airlines."
In addition, the ICAO has placed restrictions on certain combinations of letters in order to avoid confusion with other systems. For example, the designation SOS is not allowed to be used as a designator as it could be confused with the international distress signal. Other designators (such as those starting with Y and Z) are also reserved for government organizations. If an operator does not have an allocated ICAO identifier code, they are normally given the designator YYY.