the normal closed side. The common side goes to the lift pump that feeds the FPHE then to the P7100. On the Return side the Common is the two lines from the engine and the normally open goes to the diesel tank and the normally closed side goes to the veg tank.
When it is wired to work the fuel tank selector and the return valve turn on at the same time. To Flush the fuel selector turn back to diesel and the return stays in return to veg tank for the next 10 seconds.

Did I miss anything?