Canon EOS-1D X Mark III Advanced User's Manual page 338

Automatic Selection AF
The Area AF frame (entire AF area) is used to focus. AF points in focus are displayed with
< >.
With One-Shot AF, pressing the shutter button halfway displays the AF points < > that are
focusing on the subject. Display of multiple AF points indicates that each point is in focus.
Focusing areas are determined not only based on the nearest subject but also based on a
variety of other conditions such as faces and subject distance.
With AI Servo AF, you can set the initial position for AI Servo AF in [ : Initial Servo AF
pt, / ] ( ). As long as the Area AF frame can track the subject during shooting,
focusing will continue. 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.
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