A system specific unique identifier for a flight leg. Can be anything, and doesn't have to be meaningful in any way. Proposed terms will never change and will never be reused.
For instance: "YYYY-MM-DD-hh:mm:ss-xxxx-yyyyyy" where xxxx is DepartureAirportICAO and yyyyyy is FlightId or Callsign.
Another way to generate it is to use an UUID, as defined here: https://en.wikipedia.org/wiki/Universally_unique_identifier
Ole Nymoen 2020-09-21: Changed length to 36 to acomodate use of standard text UUID.