Canon EOS 5D Mark III Instruction Manual page 92

3 Selecting AI Servo AF Characteristics (For a Subject)N
AF pt auto switching

[0]
Standard setting for gradual AF point switching.
[+2 / +1]
Even if the target subject moves dramatically up, down, left, or right
and moves away from the AF point, the AF point switches to another
one to continue focusing the subject. The camera switches to the AF
point deemed most likely to focus the subject based on the subject's
continual movement, contrast, etc. Setting +2 makes the camera
more prone to switch the AF point than with +1.
However, with a wide-angle lens having a wide depth of field or if the
subject is too small in the frame, the camera may focus with the
wrong AF point.
92
This sets the switching sensitivity of the
AF points as they track the subject
moving dramatically up, down, left, or
right.
This setting takes effect in the following
AF area selection modes: AF point
expansion (Manual selection
point expansion (Manual selection,
surrounding points), Zone AF (Manual
selection), 61-point automatic selection
AF.
), AF