FAA: s regler finns i deras Platsidentifierare papper, vilket förklarar hur siffrorna används, åtminstone för USA. Det nämner också Transport Kanada, förresten, och jag antar att Kanada använder liknande regler.
Det finns särskilda krav på att en flygplats ska kvalificera sig för en trebokstavsidentifierare:
a. Three-letter identifiers are assigned as radio call signs to aeronautical navigation aids; to airports with a manned air traffic control facility or navigational aid within airport boundary; to airports that receive scheduled route air carrier or military airlift service, and to airports designated by the U.S. Customs Service as Airports of Entry. Some of these identifiers are assigned to certain staffed aviation weather reporting stations or for airports commissioning Automated Weather Observation Systems, level III (AWOS-III) or higher that have paved runways 5,000 ft or longer.
Den tre bokstäverkoden prefixas sedan med "K" för kontinentala USA, för att ge den internationella bokstaven med fyra bokstäver:
An international location indicator is a four-letter code used in international telecommunications. The location indicator for airports in the contiguous United States is the three-letter identifier preceded by "K". For other non-contiguous United States airports, the following two letter prefix will be used:
Alaska - PA, PF, PO, PP
Hawaii - PH
Puerto Rico - TJ
Virgin Islands - TI
(See ICAO Document 7910 for listings.)
Två bokstäver med två siffror används vanligtvis för offentliga flygplatser som inte kvalificerar sig för en trebokstavsidentifierare:
d. Most one-letter, two-number identifiers are assigned to public-use landing facilities within the United States and its jurisdictions, which do not meet the requirements for identifiers in the three-letter series. Some of these identifiers are also assigned to aviation weather reporting stations.
One-letter, two-number identifiers are keyed by the alphabetical letter. The letter may appear in the first, middle or last position in the combination of three characters. When the letter signifies an Air Traffic Control Center's area, the assignment will not change if the Center's boundaries are realigned.
Två bokstäver, två siffror används vanligtvis för privata flygplatser som inte kvalificerar sig för en trebokstavsidentifierare:
e. Two-letter, two-number identifiers are assigned to private-use landing facilities in the United States and its jurisdictions which do not meet the requirements for three-character assignments. They are keyed by the two-letter Post Office or supplemental abbreviation (listed below) of the state with which they are associated. The two-letter code appears in the first two, middle, or last two positions of the four-character code.