: Ensure clear identification of aircraft operators and aeronautical authorities in ground-to-ground and air-to-ground communications.

Over the North Atlantic, two flights—"Delta 123" and "DHL 123"—could be mistaken by a fatigued controller. Under ICAO Doc 8585, Delta uses DAL (telephony "DELTA") and DHL uses DHK (telephony "EXPRESS"). The unique three-letter designator ensures automation systems separate them.