Canon EOS-1D X Mark III Advanced User's Manual page 335

AF Point Expansion (Manual Selection: Surround)
Focuses using your manually selected AF point < > and surrounding AF points. Enables
focusing over a wider area than AF point expansion (manual selection ). Effective for
moving subjects, which are difficult to track with 1-point AF.
AI Servo AF and One-Shot AF operation is the same as in AF point expansion (manual
selection ) ( ).
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