Canon EOS-1D X Mark III Af Setting Manualbook page 35

Examples where the [+] setting is
effective.
The default setting is [0], which is best for shooting subjects
that move at steady speeds, or do not experience significant
changes in speeds.
[+1/+2] are best for shooting subjects that suddenly start
or stop moving, or suddenly accelerate or decelerate. This
setting enables the camera to continue focusing on the
desired subject even when it experiences significant, split-
second changes in speed. For example, the camera becomes
less likely to "focus behind" an approaching subject that
moves suddenly, and less likely to "focus in front" of a
subject when it stops suddenly, which would result in a
blurred subject. [+2] can handle greater changes in speed
than [+1].
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