The pressure regulator IS in the return circuit, as it is for almost all injection systems; in contrast with pressure regulators for carburetors, which are in the supply line.

Pressure in the working components inside the fuel distributor is maintained by bleeding off the excess on the return side. As more fuel is sent to the nozzles in response to increased demand for more power (opening throttle), a constant pressure is maintained by reducing the flow that is returned to tank. And contrariwise when demand is reduced (closing throttle).