Bosch is the original manufacturer of most of the fuel related components on your common rail Cummins, as well as numerous other diesel vehicles. The fuel rail, located on the drivers side of the engine, is responsible for holding up to 30,000 psi of fuel rail pressure delivered from the CP3 injection pump and delivering it to the six injector lines to be fed into your engine, or if sufficient fuel pressure has been reached, returning it via the pressure relief valve, also located on the rail. So why would you need to change your rail? There's a few reasons, ranging from the factory rail developing a crack, or the fact its typically less expensive to purchase the entire rail if you need the pressure relief valve (PRV) and the rail pressure sensor (RPS) separately, since those are included in this assembly.