Canon EOS 5D Mark III Instruction Manual (en)

AF Area Selection ModesN
All the AF points are used to focus. This mode is set automatically in the 
A> mode.
With One-Shot AF, pressing the shutter button 
halfway will display the AF point(s) <S> that 
achieved focus. If multiple AF points are displayed, 
it means they all have achieved focus. This mode 
tends to focus the nearest subject.
With AI Servo AF, the manually-selected (p.74) AF 
point <S> is used first to achieve focus. The AF 
point(s) achieving focus is displayed as <S>.
61-point automatic selection AF
With 61-point automatic selection AF or Zone AF, the active AF point 
<S> will keep switching to track the subject in AI Servo AF mode. 
However, under certain shooting conditions (such as when the subject is 
small), it may not be able to track the subject. Also, in low temperatures, 
the tracking response is slower.
With Single-point Spot AF, focusing with the Speedlite’s AF-assist beam 
may be difficult.
If you are using a peripheral AF point or a wide-angle lens, achieving 
focus with the EOS-dedicated Speedlite’s AF-assist beam may be 
difficult. In such a case, select an AF point close to the center.
When the AF point(s) light up, part or all of the viewfinder may light up in 
red. This is a characteristic of AF point display (using liquid crystal).
In low temperatures, the AF point’s blinking (p.74) may be difficult to see. 
This is a characteristic of AF point display (using liquid crystal).
If you set [24: Orientation linked AF point] to [
 Select separate 
AF points], you can set the AF area selection mode and manually-
selected AF point (or Zone) separately for vertical and horizontal 
shooting (p.103).
With [24: Selectable AF point], you can change the number of 
manually selectable AF points (p.100).