ICAO Three−Letter Designator (3LD)

ICAO Three−Letter Designator (3LD) − An ICAO 3LD is an exclusive designator that, when used together with a flight number, becomes the aircraft call sign and provides distinct aircraft identification to air traffic control (ATC). ICAO approves 3LDs to enhance the safety and security of the air traffic system. An ICAO 3LD may be assigned to a company, agency, or organization and is used instead of the aircraft registration number for ATC operational and security purposes.
An ICAO 3LD is also used for aircraft identification in the flight plan and associated messages and can be used for domestic and international flights.
A telephony associated with an ICAO 3LD is used for radio communication.

Source: Pilot Contoller Glossary (PCG)