Canon EOS-1DX Instruction Manual page 90

3 Selecting AI Servo AF Characteristics (For a Subject)
About the Parameters
Tracking sensitivity
[0]
Standard setting suited for most moving subjects.
[Locked on: -2 / Locked on: -1]
The camera will try to continue focusing the subject even if an
obstacle enters the AF points or if the subject strays from the AF
points. The -2 setting makes the camera track the target subject
longer than with the -1 setting.
However, if the camera focuses the wrong subject, it may take slightly
longer to switch subjects and refocus the target subject.
[Responsive: +2 / Responsive:+1]
Once an AF point tracks a subject, the camera can focus consecutive
subjects at different distances. Also effective when you want to
always focus on the closest subject. The +2 setting makes it quicker
to focus the next consecutive subject than with +1.
However, the camera will be more prone to focus on the wrong
subject.
[Tracking sensitivity] is the parameter named [AI Servo tracking
sensitivity] in the EOS-1D Mark III/IV, EOS-1Ds Mark III, and EOS 7D.
90
Sets the subject-tracking sensitivity
during AI Servo AF when an obstacle
enters the AF points or when the AF
points stray from the subject.