The Performance Development (PD) overlay zone may only be used in combination with existing conventional zones as designated herein, and the provisions of the performance development overlay zone shall become supplementary to the provisions of the zone with which it is combined. The PD zone shall not be applied to a land area as an independent zone. Property to which the PD zone has been applied shall be developed only in conformance with an approved plan. When used in combination with the designated zones, the Performance Development zone designated (PD) shall become a suffix to the designation of the zone with which it is combined and shall be shown in parentheses.