Canon EOS 5D Mark III Instruction Manual (en)

 Selecting AI Servo AF Characteristics (For a Subject)N
Tracking sensitivity
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.
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 and focus 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 
About the Parameters
[Tracking sensitivity] is the feature named [AI Servo tracking sensitivity
in the EOS-1D Mark III/IV, EOS-1Ds Mark III, and EOS 7D.