IATA is a trade association that represents its airlines around the world. Initially founded by 57 carriers, it now has some 300 member airlines from 120 countries. ICAO, on the other hand, is a specialized organization that is part of the United Nations with 193 members, with its headquarters in Montreal, Canada.
The ICAO airport codes, also known as location indicators, are the four-letter sequences used by pilots and air traffic controllers in their charts, on-board systems and in communications.
The International Air Transport Association's (IATA) Location Identifier is a unique 3-letter code (also commonly known as IATA code) used in aviation and also in logistics to identify an airport.
the letter ''K'' was simply assigned to the contiguous US by ICAO, in order to have a system with unique identifiers for world-wide use, instead of trying to adapt local system to match. The IATA codes had been in use already and possible duplicates could not be excluded.
ICAO also serves as a clearinghouse for cooperation and discussion on civil aviation issues among its 193 member-states. It is managed by a Secretariat, which is governed by a Council made up of 36 member-states, including the United States and other major actors in civil aviation.
Currently, there are 193 members or contracting states in ICAO, which is governed by the ICAO Council, which is composed of 36 member states including the United States.