Canon EOS 5D Mark III Instruction Manual (en)

Although this is the same as single-point AF, the 
selected AF point <O> covers a smaller area to 
focus. Effective for pinpoint focusing of 
overlapping subjects such as an animal in a cage.
Since Spot AF covers a very small area, focusing 
may be difficult during hand-held shooting or for a 
moving subject.
Select one AF point <S> to be used for focusing.
The manually-selected AF point <S> and adjacent AF points <w> 
(above, below, on the left and on the right) 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 an expanded AF point, 
the expanded AF point <S> will also be displayed along with the 
manually-selected AF point <S>.
AF Area Selection ModesN
Single-point Spot AF (Manual selection)
Single-point AF (Manual selection)
AF point expansion (Manual selection  )