is the official Location Indicators manual published by the International Civil Aviation Organization (ICAO). This technical publication serves as the definitive global registry for the four-letter codes used to uniquely identify aerodromes, air traffic control centers, and other aeronautical facilities worldwide. Overview of Doc 7910
Do not confuse the two. Flight plans require both: a location indicator (Doc 7910) for departure/arrival and an aircraft operator designator (Doc 8585). doc 7910 pdf