The rules that use the category "<digit-pl>" are:

<flight_id-not_creatable-pl>

<flight_noun-pl> of <digit-pl> <specific_aircraft_type-pl>

<up_to_two_digits-pl>

<digit> <digit-pl>

<flight_id-not_creatable-sing>

<flight_noun-sing> of <digit-pl> <specific_aircraft_type-pl>

<flight_id-creatable-sing>

<flight_noun-sing> of <digit-pl> <specific_aircraft_type-pl>

<flight_id-not_creatable>

<flight_noun> of <digit-pl> <specific_aircraft_type-pl>