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

<unit_call_sign>

<air_call_sign_prefix>

<air_call_sign_prefix> <air_call_sign_designator>

<air_call_sign_prefix-takes_digit> <digit>

battalion <digit-not_zero>

battalion <digit-not_zero> <digit>

battalion <digit-not_zero> <digit> <digit>

battalion <digit-not_zero> <digit> <digit> <digit>