The rules that use the category "<any_letter>" are:

<name_operation>

<naming_verb> it <any_letter>

<unit_call_sign>

<any_letter> <digit> <any_letter>

<point_id-waypoint>

<any_letter>

<point_id-modloc>

<any_letter>

<point_id-objective>

<any_letter>

<point_name_mod>

<name_adj> <any_letter>