For the purposes of this Schedule the extent to which subjects are used residentially shall be determined by reference to the use made of the subjects as the sole or main residence of any person, and criteria may be prescribed by reference to which any apportionment or re-apportionment of net annual values and rateable values under this Schedule is to be carried out.

No rates shall be leviable in respect of such part of their rateable value as relates to the residential use of any lands and heritages which are part residential subjects.