Canon EOS 5D Mark III Instruction Manual (en)

AF Area Selection ModesN
The manually-selected AF point <S> and adjacent AF points <w> are 
used to focus. The AF point expansion is larger than with AF point 
expansion (Manual selection 
), so the focusing is executed over a 
wider area. Effective when it is difficult to track a moving subject with 
just one AF point.
AI Servo AF and One-Shot AF work in the same way as with AF point 
expansion (Manual selection 
) mode (p.75).
The 61 AF points are divided into nine zones for focusing. All the AF 
points in the selected zone are used for the automatic selection of the 
point of focus. It makes achieving focus easier than with single-point AF 
or AF point expansion and it is effective for moving subjects.
However, since it is inclined to focus the nearest subject, focusing a 
specific target is harder than with single-point AF or AF point expansion.
The AF point(s) achieving focus is displayed as <S>.
AF point expansion (Manual selection, surrounding points)
Zone AF (Manual selection of zone)