When to use them is also based on experience. The booster banjo on the left has a semi circle cut into the bottom of it. This is an anti-reversion device. It also creates a vortex swirl and really aids in acceleration on some applications. But once again, you can't use this option all the time.
Notched floats are a necessity.
These notches allow the float to be stabile, drop properly and allow fuel to refill the bowl to keep the engine from going lean. Aaaah, when to plug an emulsion and when to enlarge them. That choice varies with the powerplant and HP level in relation to signal created and fuel consumed. A simple brass plug in the right spot can realize 10-15 lbs of torque at peak. This is where carburetors are at in the year 2001.