Both, its just a t that ties the Cp3 return fuel, and rail fuel to the main retun line that goes to the tank. In your case I wouldnt mess with any of the factory fuel lines and just use a pump and solenoid valve to add bio or whatever you are using to the factory tank when you want to fill it. I have an aux tank on my truck and when I want to fill the main tank I just flip a switch that turns on the pump and opens the valve and fills through the vent line on the factory fuel fill.