Canon EOS-1D X Mark III Advanced User's Manual page 366

Parameters
Tracking sensitivity
Setting for AI Servo AF/Servo AF subject-tracking sensitivity in response to non-subjects
moving across AF points or subjects straying from AF points.
0
Standard setting. Suitable for moving subjects in general.
Locked on: –2 / Locked on: –1
The camera will try to continue focusing on the subject even if non-subjects move
across AF points or if the subject strays from the AF points. The –2 setting makes the
camera keep tracking the target subject longer than the –1 setting.
However, if the camera focuses on a wrong subject, it may take slightly longer to switch
and focus on the target subject.
Responsive: +2 / Responsive: +1
The camera can focus consecutively on subjects at different distances that are covered
by the AF points. Also effective when you want to always focus on the closest subject.
The +2 setting is more responsive than the +1 setting when focusing on the next
subject.
However, the camera will be more prone to focus on an unintended subject.
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