From the main engine inlet rail the fuel enters each fuel pump (one per cylinder). The fuel pump delivers the fuel at high pressure (250bar +) to the fuel injector(s).

The lower connection on the fuel pump is the fuel return. More than the required fuel is delivered to the  pump. The excess is recirculated back to the buffer (mixing) tank.

The fuel injectors deliver the atomised fuel to the cylinder. When not injecting fuel, the injectors recirculate back to the fuel return