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

Double-sided forced sealing for gate valves:

This means that whether at the inlet or outlet end of the medium, the gate plate and valve seat sealing surfaces remain sealed. The sealing integrity is forcibly maintained by the axial force of the valve stem. When no medium is present, the positive pressure between the sealing surfaces must not be less than the sum of the medium's static pressure and the sealing force.

Single-face forced sealing for gate valves:

This means that no seal exists between the gate plate and the valve seat sealing surface at the medium inlet end. Here, either no pressure differential exists at all, or the pressure differential is less than the sealing pressure. On the medium outlet side, the seal between the gate plate and the valve seat sealing surface is forcibly maintained by the axial force of the valve stem and the medium pressure. When no medium is present, the pressure differential across the sealing surfaces must not be less than the sealing pressure.