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

<up_to_two_digits-sing>

<digit> <digit-sing>

<degree_number-not_initial_zero-90-not_digit_sequence-sing>

<digit-sing>

<second_setting>

<digit-sing> <time_interval_unit-second-sing>

<number-sing>

<digit-sing>

<minute_setting>

<digit-sing> <time_interval_unit-minute-sing>

<degree_number-180-not_digit_sequence-sing>

<digit-sing>