Canon EOS-1D X Mark III Advanced User's Manual page 337

Large Zone AF (Manual Selection of Zone)
The AF area is divided into three focusing zones (left, center, and right) for focusing. Covers
a larger area than Zone AF, using all AF points in your selected zone 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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