Canon EOS-1D X Mark III Af Setting Manualbook page 24

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 indicated above.
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