Canon EOS-1D X Mark III Advanced User's Manual page 863

: Switch to Registered AF Point
Pressing the button assigned to this function during metering enables you to switch to the
registered AF point.
* 3: To select [Switch only when btn is held] or [Switch each time btn is pressed], press the
< > button when the setting screen is displayed. For instructions on registering AF points,
see Registering the AF Point.
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