That fitting is connected to the hoses that go into the top of the float bowls of the carburetor. Basically, at higher speeds, the ram air system allows that line to pressurize the float bowls for more fuel pressure. The T fitting that is in question is there to allow the carbs to vent any excess overflow of fuel from the float bowl instead of back into the airbox. It should stay unplugged as you see it, and don’t freak out if you ever see a tiny bit of fuel dripping out of it, only if it is for some reason spewing fuel should you be concerned(most likely a stuck float or faulty seal on a float needle allowing fuel to continuously load into the carb.) hope this helps some!