Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 326)

Selecting the AF Area Selection Mode
1.
Press the < > button ( ).
2.
Press the < > button.
Look through the viewfinder and press the < > button.
Each time you press the < > button, the AF area selection mode
changes.
Note
With [ : Select AF area selec. mode ], you can limit the selectable AF area
selection modes to only those you want to use ( ).
If you set [ : AF area selection method ] to [ → Main Dial ], you can select
the AF area selection mode by pressing the < > button, then turning the
< > dial ( ).
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