BEGINNER using Durso overflow config.

In my new tank set up, I have a center overflow with three holes. I want to use the Durso method. So... (1) hole for the Durso, (1) hole for the return line and (1) for...... what? Is this for an emergency overflow pipe just below the surface? I see this in the Herbie style but I am too much of a newbe to know if this is "standard" or part of the Durso. If no, what do I use the third hole for? Thank you very much