Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 381)

Select AF area selec. mode
You can limit the selectable AF area selection modes to suit your shooting preferences.
Select the desired AF area selection mode and press <
> to add a checkmark [ ]. Select
[ OK ] to register the setting. The AF area selection modes are described on AF Area
Selection Modes (Viewfinder Shooting) .
Caution
The [ ] mark cannot be removed from [ Manual selection:1 pt AF ].
Even if you add a checkmark [ ] in [ Select AF area selec. mode ], some AF area
selection modes are not available with certain lenses.
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