Pressure p A or p B in the operating ports A or B with a load that is not subjected to load and hydraulic zero. The clamping pressure is then equal to half the sum pressure. This statement does not apply for proportional pressure valves! (DIN 24 311).
The clamping pressure arises due to the throttling of the flow rate at the control elements (nozzles, edges of the valve control spool/slot) in the working lines to an unloaded, double acting cylinder or hydraulic motor.