Something that no one has mentioned yet is the bubble on one side of the tank. That is there for the vent tube to go into so you can completely fill the tank IF the tank is installed upright. Just for sake of argument, when I install a fuel tank in a boat, I put the vent in the top right corner and the fuel pick up in the bottom left so I can totally empty the tank. This only works since the tank is installed on a sloped surface so the top is at the front of the boat and the bottom is at the rear. Since I don't have to worry about banking or inverted flight, I actually get to use hard pick up lines inside the tank where, in an aircraft, the clunk type pick up is almost mandatory