Canon MARK IV User Manual (en)

Custom Function Settings
2: Release/Drive speed priority
For the first shot, shutter release is given priority over focusing the 
subject. During continuous shooting, the continuous shooting speed is 
given priority more than with setting 1.
3: Release/Tracking priority
For the first shot, shutter release is given priority over focusing the 
subject. For the second and following shots during continuous shooting, 
focus-tracking of the subject is given priority.
C.Fn III -4     AI Servo AF tracking method
In the AI Servo AF mode while you are focus-tracking a subject, the camera 
can either continue focusing the target subject even if a closer subject 
(closer than at the main focus point) suddenly appears in the picture, or the 
camera can switch to focus the closer subject.
* Main focus point = With automatic AF point selection: Center AF point
With manual AF point selection + AF point expansion (C.Fn III -8-
1/2/3): Manually-selected AF point
0: Main focus point priority
The active AF point will switch to the main focus point and start focusing 
the closer subject. Convenient when you always want to focus the 
closest subject.
1: Continuous AF track priority
Any closer subject appearing in the picture will be ignored as an 
obstruction. The main focus point does not take priority, so the tracking 
of the target subject can continue and switch to an adjacent AF point 
based on the preceding focusing result. Convenient when obstacles 
such as telephone poles appear in front of the target subject.
C.Fn III -5     Lens drive when AF impossible
If focus cannot be achieved with autofocus, the camera can either keep 
trying to focus or stop.
0: Focus search on
1: Focus search off
Prevents the camera from becoming grossly out of focus as it attempts 
to focus again. Especially convenient with super telephoto lenses which 
can become extremely out of focus.