Canon EOS-1D X Mark III Advanced User's Manual page 378

Subject switching
• Disable
Tracks the subject initially determined for AF, to the extent possible.
• Enable (slow)
Generally tracks the subject initially determined for AF. May switch to other subjects
if the current subject is no longer suitable for AF.
• Enable
Switches to track other subjects in response to shooting conditions.
Caution
Precautions when set to [Disable]
The camera may not be able to continue tracking subjects under some subject
conditions.
• If subjects move vigorously
• If subjects face a different direction or change their posture
• If AF points stray from subjects, or if subjects are hidden behind
obstructions and you can no longer see them
With [AF priority (People)] set to [Enable], the camera may switch to the face
or head, depending on subject conditions.
Subjects selected by touch will be tracked to the extent possible regardless of
the option set in [Subject tracking settings].
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