Purpose of evap valve under the intake manifold.

[san rensho]

I'm trying to chase down a vacuum leak in the evap system and I'm having trouble figuring out how the system works.

Near the carbon canister, there is a "tank vent valve" (also called a purge valve) that is controlled by the DME to purge the carbon canister of fuel vapor under certain conditions. The TVV works in conjunction with another valve near it, the "shut off valve" which opens and closes to let fresh air into the system when the TVV is open and purging the carbon canister.

So what is the purpose of another valve (I don't even know its name) under the intake manifold thats in the same circuit as the TVV?