Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 382)

AF area selection method
You can set the method for switching AF area/AF method selection modes.
[ ] →M-Fn button
After you press the < > button, pressing the < > button will change the AF
area/AF method selection mode.
[ ] →Main Dial
After you press the < > button, turning the < > dial will change the AF area/AF
method selection mode.
Note
When [ →Main Dial ] is set, use < > to move the AF point horizontally.
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