Canon EOS-1D X Mark III Af Setting Manualbook page 32

An example where [Locked on: -2/1] is more
effective.
When another player crosses in front of the subject, focus
can shift to the foreground/background.
Example where a player momentarily comes in front of
the player being tracked, and the target player reappears.
With the [Locked on: -1] setting, the overlapping player is
not focused on, and it is possible to continuously track the
original target player.
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