Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 334)

AF Point Expansion (Manual Selection )
Focuses using your manually selected AF point < > and surrounding AF points. Effective
for moving subjects, which are difficult to track with 1-point AF.
Although with AI Servo AF, the camera must be able to focus on your initially selected AF
point <
>, this mode makes it easier to focus on targeted subjects than with Zone AF.
With One-Shot AF, after the camera focuses with expanded AF points, those AF points
<
> are displayed with your manually selected AF point < >.
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