Canon EOS-1DX MARK II Settings Manual page 62

The wider area covered by AF Point Expansion means
that even if a subject moves away from the central,
primary AF point, the outer points are available to
continue to focus upon it. This high jump athlete was
captured with [AF point expansion (surrounding)] —
that is, eight surrounding "assist" points .
Hints & Tips
[Up, down, left, and right] and [surrounding]
options can be selected according to the
difficulty of reading the movement, and the
relative importance to the central AF point
When shooting subjects which might be difficult to
determine movement, select [surrounding] mode,
and when you expect you can normally keep the
central point upon a moving subject, it may be
preferable to select the [Up, down, left, and
right] (4-point expansion) mode.
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