Canon EOS-1DX MARK II Settings Manual page 44

Set how rapidly the AF point switches
to a new AF point for moving subjects
The [AF pt auto switching] parameter is only for
switching between multiple AF points, so it is unavailable
when using AF area selection modes [Single-point Spot
AF] and [Single-point AF].
The [0] setting is a standard setting for moderate
switching of AF points. The [ + 1/ + 2] settings are used
when shooting subjects with erratic movement which
could happen in any direction. When an initial AF point
(AF point being focused during Auto selection AF, Large
Zone AF, Zone AF) leaves the subject, it will aggressively
switch to surrounding AF points to capture the subject.
Use the [+] setting when you want the camera to
automatically decide (switch) to use a new AF point, and
the [0] setting is used when you want to place emphasis
on the initial AF points being used to track the subject.
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