Canon EOS-1D X Mark III Advanced User's Manual page 344

: Large Zone AF: Horizontal
Uses auto selection AF in a horizontal Large Zone AF frame to cover a larger area than
Zone AF, which makes focusing easier than with 1-point AF/AF point expansion and also
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.
AF points in focus are displayed with < >.
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