In this photo you can see the black 90 degree fitting I snapped on. That is the main vent port out of the fuel tank. The other smaller port goes into the same chamber so you can use either of these. For me, I hooked my vent up to the 2009 Chevy Evap canister which is this size of hose. I also teed into the factory Blazer vent which is up in the filler neck. That way I would hopefully have less issues filling the tank.
As I said this line runs to the Evap cannisters, from there it is controlled by the ecm to purge into the engine. It also has a fresh air vent on there with a vent control solenoid. Basically all this is to keep the truck from smelling up my garage with gas fumes! Nothing I hate more than a truck that stinks up your garage. I always find it funny when people rip this stuff out, not even knowing what it's function is.
Also, here is a picture of my Evap canister mounted under the driver's side of the truck.
