IATA based identifier for this flight, usually issued long before the flight actually takes place.FlightId is normally the concatenation of OperatingAirlineIATA, FlightNumber and OperationalSuffix.
FlightId typically identifies a flight to the majority of systems, but it is not unique across time. It's unique only in conjunction with FlightDepartureDate.
Exception: Some airlines use their ICAO code (OperatingAirlineICAO) instead of OperatingAirlineIATA. This might be because they aren't an IATA member or because they just prefer the ICAO code. Regardless, this means that it is allowed to use OperatingAirlineICAO as part of FlightId.
FlightId is then defined as the concatenation of AirlineIATAorICAO, FlightNumber and OperationalSuffix.
