If you mean the one pointing to the left that's nominal AOA, ie none. The aircraft datum will line up to that spot when you're on the ground. Bottom mark is min AOA (about -5) and the top one is max, 24. I was getting good results landing around 10 AOA but not sure if that's correct. But that's about half way from the middle notch and the top. Anyways whatever AOA you want it's a nice reference to have and made landings a lot easier.