Canon EOS-1D X Mark III Af Setting Manualbook page 51

Using this setting shifts the focus point used from a manually
selected AF point, to an adjacent (up, down, left, and right,
or around) AF point, when required to aid focus tracking.
When using this setting it is easier to obtain the desired
composition as the subject is captured centering around
the manually selected AF point.
Based on the subject's movement characteristics, (i.e.
likelihood of subject moving from the selected AF point)
and the size of the subject within the frame, select either
[AF point expansion ( )] or [AF point expansion (Around)].
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