Canon EOS-1DX MARK II Settings Manual page 21

In other shooting situations, this setting can be extremely
effective when switching between different subjects you
want to shoot (for example, at the start of a bicycle road
race, when you want to shoot continuously and switch
from cyclist to cyclist while focusing).
When set to Case 3, if the subject moves away from
the AF points, the camera may quickly refocus on a
different subject or background, in contrast to Case 2.
Therefore, it is recommended that you use this setting
only when you have a particular objective as described
above.
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