Canon EOS-1D X Mark III Af Setting Manualbook page 54

Zone AF
With the [Zone AF] mode, one AF point
is selected from multiple AF points, and
the AF point is automatically selected
from within that zone.
It selects an AF point in a wider range
than 1-point or AF point expansion
during Live View shooting.
Effective for capturing subjects within
a selected zone.
Viewfinder shooting
[Zone AF
(Manual selection of zone)]
The selected AF points display in [Zone AF].
Zone AF differs from [AF point expansion], which where AF
tracking is based around the manually selected AF point.
With Zone AF the camera decides the position to focus on
the subject within the zone, so it is effective for situations
when focusing with either [1-point AF] or [AF point
expansion] is difficult.
Zone AF is the best option when you want to focus on areas
of the subject that are a larger size.
Live View shooting
[Zone AF]
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