Canon EOS 5D MARK III Instruction Manual page 22

,_,2: 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
AF.
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.
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