Canon EOS-1D X Mark III Af Setting Manualbook page 58

Selection can be made from three focusing
zones during viewfinder shooting.
With the optical viewfinder the AF points are divided into
three main zones, left, center, and right, and auto selection
of AF points takes place within the selected zone.
Viewfinder shooting
During Live View shooting, [Large Zone AF (vertical)] and
[Large Zone AF (horizontal)] can be selected.
Live View shooting
When using
[Large Zone AF (horizontal)]
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