Canon EOS-1DX MARK II Settings Manual page 70

With [AI Servo AF], the camera normally begins focusing
on a moving subject at a user-defined single AF point,
and will then change AF points automatically to follow
the subject if it moves around the frame. However,
depending on the shooting conditions or if the subject is
small, this side-to-side tracking around the AF array may
not be possible, so caution is necessary.
[Auto selection AF] is a great option for a sequence of
shots, where you want a moving subject to move across
the frame, from shot to shot. It can also be useful simply
to keep track of subjects which will move erratically
(figure skating jump scenes for example). Another
effective use is when you want to take action images for
publication/articles with lots of space in the composition
for text etc.
In the examples below, shooting began by capturing
the targeted cyclist first with a manually selected AF
point (in the center etc.). From there, while continuously
shooting and moving the camera (lens) to the left or
right, it is possible to position the lead cyclist off to one
side and include a lot of background (focusing continues
to track the cyclist by automatically switching AF points).
[Auto selection AF] can be effective when you want to
include both the subject and the background in the
composition when shooting.
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