It's the shutoff from the fresh tank. Use it to Winterize. Drain the fresh tank, close that valve, then you use the Winterize hose into antifreeze. This valve keeps the pump from pulling from the fresh tank and forces it to pull from the siphon hose (bottom center of picture-pull hose out, remove cap and stick in gallon jug). I have the same setup. It will not fill the fresh from the city water connection. Valve has to be open to pump water from fresh tank. Otherwise, it can stay closed all the time. , .