AF Area Selection Modes 
The AF areas are explained starting with the smallest one.
(1) Spot AF (Manual selection) C.Fn III -6
Although this is the same as single-point AF, the selected AF point <O> covers a smaller pinpoint area to focus. Effective for pinpoint focusing overlapping subjects such as an animal in a cage. Since Spot AF covers a very small area, focusing might be difficult during hand-held shooting or for a moving subject.
(2) Single-point AF (Manual selection)
Select one AF point <S> to be used to focus.
(3) AF point expansion (Manual selection) C.Fn III -6
The manually-selected AF point <S> and adjacent AF points <w> are used to focus. Effective when it is difficult to track a moving subject with just one AF point.
With AI Servo AF, the manually-selected AF point <S> must focus- track the subject first. However, it is easier to focus the target subject than with Zone AF.
With One-Shot AF, when focus is achieved with the expanded AF point, the expanded AF point <S> will also be displayed along with the manually-selected AF point <S>.