The picture of the 3 line tank plumbing has 3 labels for the lines.

The top label says VENT. Most glow engines nowadays run best with muffler pressure. This line is very often connected to the pressure tap on your muffler. When you fuel the airplane using the line labeled Fuel Dot, you discover that the tank is full when fuel overflows out that VENT line, wherever it goes. If it can quickly and easily get out of the muffler, you have no problems seeing that the tank is full just as soon as it is full. If the muffler layout doesn't do that, many people put a T in the vent line and run the extra line to wherever the Fuel Dot line goes. They have two lines sticking out somewhere and both are plugged. The attached picture shows how I do most of my airplanes.

The two lines are pushed into the cowl in the picture. That's how they are except when fueling. Pull them both out and remove the plugs to fuel. Into the front one that goes where the Fuel Dot line in RCKens picture goes, and when full, you'll see overflow out the rear one.

works like a charm

The T's available at most LHSs are absolutely adequate and dependable with today's excellent silicon fuel tubing.

The plugs are sold in packs of two. They do what Fuel Dots do. There is really little difference in function or reliability. Both the plugs and dots are as reliable as your ability to stick a plug into a hose.