It's a vent so vacuum don't build up in the tank and stop the fuel from flowing. Its a gravity system, while Walbros pump there fuel out of the tank. The stock black plastic gas caps with a tiny hole don't work very well. On my avatar bike the line to the cap is the return fuel line from the Walbro 603.
the way ive done this in the past (different from how others might go about doing this) is drilled a hole in the center of the cap and threaded in a male hose fitting and then ran the hose that attaches to the fitting wherever I wanted.
i run mine thru bar hole on left - straight down couple inch to like a square hole on the frame i bend it thru. ill take a pic sometime - theres a tophat shape nipple ya pop off and drill then i just drilled str8 thru and put the hat piece back. mine does work
all mine cost me was 5 minutes of picking a color of aqarium hose, drilling like someone suggested in a searched for thread (hint hint), and praying that it would work.