The valve is to be used for a water distribution network to reduce high system pressures downstream of the valve. "They" refers to the tech. specification of the valve produced by the designer.
If you want to reduce the pressure downstream to a preset value, the valve is a pressure regulator and the pressure tap (internal or external) is locate downstream. Lots of manufacturers provide this type of PRV ; see Emersonprocess (formerly Fisher) model 95 for example.
If you want to control a constant flow rate donwstream independently of upstream pressure, the valve is a differential pressure regulator ; 2 pressure taps is needed (upstream and downstream). See Jordan valves (model 63).