• "SCH" - Scheduled
  The initial state for scheduled flights.
  A known FlightLeg that is expected to depart and land according to the available information.
  Can transition to: FPL, ACT, CAN

• "FPL" - Flight Plan
  Initial state for flights that aren't scheduled.
  The flight plan has been filed and is registered by ATM. 
  Can transition to: FLS, SEQ, ACT, CAN

• "FLS" - Flight Suspended
  The flight has been suspended by ATM, but can be activated at any time. 
  Can transition to: FPL, CAN,

• "SEQ" - Sequenced
  The flight has been sequenced by ATM. 
  Can transition to: ACT, CAN

• "ACT" - Active
  The aircraft is “off block”, and might be airborne.
  Can transition to: LAN, RER

• "CAN" - Cancelled
  The FlightLeg has been cancelled. 
  End state.
  

• "LAN" - Landed
  The aircraft has landed on the scheduled airport.
  End state.

• "RER" - Rerouted
  The aircraft is in the air, but the destination airport has been changed.
  After landing the state will change to “DIV”.
  The destination airport can be changed back to the original airport. It is then a regular flight leg, but possibly delayed.
  Can transition to: DIV, ACT

• "DIV" - Diverted
  The aircraft has landed on an unscheduled airport.
  End state.

• "DEL" - Deleted
  The FlightLeg appears to have been deleted/removed.
  This is probably the result of missing schedule change information or some other data errors.
  End state.
  
  
• "UKN" - Unknown
  Unknown indicates missing information.
  

A transition to DEL (if used) and UKN can in theory happen from all states. For this reason the transitions aren't shown in the state diagram below.

• "UKN" - Unknown
  Unknown indicates missing information. Usually this means that it’s time based when a state transition to unknown.
• "DEL" - Deleted
  It is a matter of system design if this state is used. Deleted flights can be physically deleted (in databases), but they can also be marked as deleted and become "invisible". If “DEL” is used as a status then it is set when information arrives that explicitly deletes the flight leg, or when it can be deduced that it is an erroneous entry that should be deleted (ghost flights for instance).
• "CAN" - Cancelled
  A cancelled flight will often not bed deleted, but only marked as cancelled. There are IATA messages that can cancel a flight (leg) and then reinstate it later.

2020-12-04: Description updated, Comments updated.
2021-02-11: Transition from RER to ACT added.