Canon EOS 5D Mark III Instruction Manual (en)

82: Custom Controls
The AF will stop while you hold down the button assigned to this 
function. Convenient when you want to lock the focus during AI Servo 
After setting AF area selection mode (p.72), Tracking sensitivity (p.92), 
Acceleration/deceleration tracking (p.93), AF point auto switching 
(p.94), AI Servo 1st image priority (p.96), and AI Servo 2nd image 
priority (p.97) and assigning this function to a button, you can apply 
these settings while you hold down the assigned button for AF. 
Convenient when you want to change the AF characteristics during AI 
Servo AF.
*2: In step 3 on page 327, if you press the <
button, the “Switch to registered AF func.” setting 
screen will appear. Turn the <5> or <6> dial to 
select the parameter to be registered, then press 
0> to append a checkmark <X>. When you 
select a parameter and press <
0>, you can set 
the parameter.
By pressing the <L> button, you can revert the 
settings to their defaults.
You can switch the AF mode. In One-Shot AF mode, when you hold 
down the button to which this function is assigned, the camera switches 
to AI Servo AF mode. In the AI Servo AF mode, the camera switches to 
One-Shot AF mode only while you hold down the button. Convenient 
when you need to keep switching between One-Shot AF and AI Servo 
AF for a subject that keeps moving and stopping.
: AF stop
: Switch to registered AF function