By registering the AF point you often use, you can switch to it instantly. You can register an off center AF point which suits your preferred framing or automatic selection for the shooting moment priority. Any of the seven AF points can be registered. Only one AF point can be registered
SWITCHING TO THE REGISTERED AF POINT
Normally, to switch to the registered AF point, you press the <> button and <> button simultaneously. However, with C.Fn-13-1/2, you can just press the < > button to switch to the registered AF point.
If C.Fn-13-1/2 and C.Fn-04-1/3 have been set, you can press only the <> button to switch to the registered AF point and start autofocusing at the same time.