Canon EOS-1D X Mark III Advanced User's Manual page 336

Zone AF (Manual Selection of Zone)
The AF area is divided into nine focusing zones for focusing. All AF points in your selected
zone are used for automatic selection AF, which makes focusing easier than with 1-point AF
or AF point expansion and effective for moving subjects.
Focusing areas are determined not only based on the nearest subject but also based on a
variety of other conditions such as faces, subject motion, or subject distance. Note that
faces of any people in the Zone AF frame are given priority in focusing.
AF points in focus are displayed with < >.
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