I actually didn't describe my plumbing setup because I use two different ones. I use uniflow most times, but will use what RCKen's picture shows for airplanes I'm not going to stunt much.

When you have all of your fuel lines enclosed and unreachable inside a cowl, you have to make at least one of them available to the outside world. You need the fill line available. A plugged line that comes through a cowl hole or a fuel dot on the cowl itself work for making that fill line available. But if your muffler isn't in the right place to show you overflow immediately, then you're going to need two lines available to the outside world. And that second line can be plugged or dotted too.

How does the T line work? Great. It's only use is to show the tank overflow. And that won't happen except when the tank fills and fuel starts to flow out the overflow. Yes, the overflow goes to the muffler and out the T line, but as soon as you see that fuel you stop fueling. It isn't a big deal. And if you want, you can arrange the T in the line so the overflowing fuel's momentum goes straight through the top of the T to the outside world instead of the muffler. Works great and the muffler pressure doesn't care if it's got to make a right angle turn.

With either uniflow or conventional when you have two lines showing to the outside world, you unplug them both to fuel. One will be the fill and must always be used for that. You unplug both lines and fill into the correct one. When the overflow appears out the overflow line, you stop pumping and plug both lines.