In Leon it is possible to add Hold Item List to the aircraft, in section Maintenance > Hold Item List (more details about HIL section can be found here ).
If you have added an open HIL to one of your aircraft within a certain period of time, in section PLanned Flights su ch aircraft appears in orange (see screenshot on the right).
If you try to add a new flight from Planned Flights section, Leon will issue a warning:
Aircraft A-BCDE has a hold item list open in section “Maintenance > Hold Item List”
The same applies if you add a new flight from New Flight section.