Application of Income-tax provisions to interest-tax, with necessary modifications and assessee defined under the Act. Section 21 provides that specified sections and Schedules of the Income-tax Act and the Income-tax (Certificate Proceedings) Rules, 1962, shall apply to the Interest-tax Act with necessary modifications to refer to interest-tax, and that references to 'assessee' in those provisions are to be construed as references to an assessee as defined in the Interest-tax Act.