In general:
Sum of pressure in the operating ports of a (continuously adjustable directional valve, hydraulic motor).
For valves:
Sum of pressure p A and p B in the operating ports A and B with a consumer that is not subjected to load and hydraulic zero (DIN 24 311). The sum pressure gives an indication of the actual overlap/underlap conditions present in the valve (Figure S 111).
Figure S 111: Sum pressure at exactly zero (a), zero outside and negative inside (b), negative outside and zero inside(c)