The category "<unit_call_sign-gapsout_null-gapsin_null>" can be expressed as follows:

<unit_call_sign-gapsout_null-gapsin_null>

<air_call_sign_prefix>

<air_call_sign_prefix> <air_call_sign_designator>

<air_call_sign_prefix-takes_digit> <digit>

<dynamic_unit_call_sign>