Canon EOS-1DX MARK II Settings Manual page 119

AF area selection modes such as [Single-point AF] and
[AF point expansion] are effective for switching
according to the size and type of subjects. While looking
through the viewfinder it can be difficult to change the
mode while tracking the subject. However, by assigning
the AF area selection modes you want to use to specific
buttons, you can switch instantly while continuing
shooting.
There are five buttons on P. 117 that can be assigned.
Think about the characteristics of the sports and subjects
you want to shoot beforehand, and assign the AF area
selection modes you think you will use.
By assigning different modes to each of the five
buttons, you can control up to five modes at will. In
addition, aside from the AF area selection modes, various
functions can be registered and applied, so by making
settings as needed, you have the flexibility to handle
conditions as they change.
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