FlightLegIdentifier is a set of attributes that uniquely can identify a FlightLeg in different contexts. It consists of the following:

• IFPLID
• Callsign
• AircraftRegistration
• SSRCode
• FlightId
• FlightDepartureDate
• DepartureAirportIATA
• ArrivalAirportIATA
• DepartureAirportICAO
• ArrivalAirportICAO

None are required, but enough have to be present to actually uniquely identify a FlightLeg.