In a bean animal overflow box you have 3 pipes. The shortest pipe should be your full siphon, and also the one with a gate valve on it. The next tallest pipe is your first emergency, and should handle small fluctuation is water height. Third pipe is the tallest and should remain dry, unless the other 2 become blocked.
With an AC pump, you adjust the height in the overflow box, with the gate valve. This is what we call "tuning" the system. It should be set so the water in the overflow box is just at the top of your first emergency pipe.
With a DC pump the idea is exactly the same, except you can adjust the pump instead of adjusting the gate valve. But with a DC pump you can set it lower and open up the valve more, or vice versus. It's all on how you want to "tune" the system.
The third pipe is only there if there is a blockage in the other 2 pipes.