As far as I can tell from some quick research on the 'net, it's part of the diesel injection system that returns unused fuel to the tank.
Modern common rail diesel injection systems run at a very high pressure that has to be maintained between tight limits. To do this they use a circulation system of a high pressure pump, a common "rail" which all the injectors are connected to and valves to regulate the pressure. Fuel that's not used by the engine ends up in the spill rail and is returned to the tank. It's this "spill rail" that has been found to develop leaks.
Mine's booked in on Thursday for the recall. The dealer was unaware of the problem until they checked Topix