For gate valves, what is a double-sided forced seal? What is a single-sided forced seal?

Double-sided forced sealing in gate valves:

This means that the gate and the seat sealing surfaces are sealed at both the inlet and outlet ends of the valve. The seal is maintained by the axial force of the valve stem. When no medium is present, the normal force between the sealing surfaces must not be less than the sum of the static pressure of the medium and the sealing force.

Single-sided positive sealing in gate valves:

This means that there is no seal between the gate and the seat sealing surface at the medium inlet; in this area, there is either no contact pressure at all, or the contact pressure is less than the sealing contact pressure. On the medium outlet side, the seal between the gate and the seat sealing surface is forcibly maintained by the axial force of the valve stem and the medium pressure. When there is no medium present, the contact pressure on the sealing surface must not be less than the sealing contact pressure.