It's the line to the canister. The 3rd line is blank...none of my Jeeps have had a 3rd line.
There are two vents on top of the fuel tank, they each have a rubber vac hose that then comes together in a T, then the one single hose goes to a metal vac line. It is metal from there to close to the charcoal canister, then it goes back to rubber and Into the canister. The bigger of the two lines from the canister is the source vacuum from the intake/manifold. The "system" sucks fuel tank fumes through the charcoal canister and then in to the intake/manifold to be burned off.
It's an emissions thing. In CA they test the fuel tank to assure that the vents on the tank are connected to something. The valves on the tank are also known as "roll-over" valves, so if your Jeep is upside down it won't leak fuel. Is it necessary? In CA, yes...in my county anyway...and I live in the sticks.
If your area doesn't test for emissions or has looser regulations it probably isn't necessary and probably doesn't make it run any different...EXCEPT for there being a loose vacuum hose flopping around.
Than there's the whole thing about fuel leaking if you're upside down
1990 XJ Laredo - White & Chrome
1988 MJ Pioneer - Metallic Green and Black