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

<call_sign_prefix>

<any_letter>

<point_id-objective>

<any_letter>

<id_letter-sing>

<any_letter>

<any_letter> and <any_letter>

<ucs_letter>

<any_letter>

<name_operation>

<naming_verb> it <any_letter>

<unit_call_sign>

<any_letter> <digit> <digit>

<point_name_mod>

<name_adj> <any_letter>

<id_letter-pl>

<any_letter> and <any_letter>