IATA airline designators, sometimes called IATA reservation codes, are two-character codes assigned by the International Air Transport Association (IATA) to the world's airlines.
In this case, an IATA airport code for bookings of passengers, consisting of three letters is used (although in SL, and effects to the pilots always retain ICAO code).
The standard is described in IATA's Standard Schedules Information Manual and the codes themselves are described in IATA's Airline Coding Directory... More info here.