Canon EOS-1D X Mark III Advanced User's Manual page 912

Group H
The 23 AF points shown here are available for autofocusing. As the AF area selection mode,
choose from Spot AF, 1-point AF, Expand AF area:
point Automatic selection AF.
With an extender attached to extendable lenses, AF is available when maximum aperture is
f/8 (in a range of f/5.6–f/8).
: Cross-type AF point. Offers excellent subject tracking and high-precision focusing.
: AF points sensitive to horizontal lines (AF points left and right of the center) or vertical
lines (AF points above and below the center).
: Disabled AF points (not displayed).
Caution
Expand AF area:
selected AF points that are not surrounded by active AF points is limited to any
available adjacent points.
If the maximum aperture value is slower than f/5.6 (greater than f/5.6 but not
exceeding f/8), focus may not be achieved with AF when shooting low-contrast or
low-light subjects.
If the maximum aperture value is slower than f/8 (greater than f/8), AF is not
possible during viewfinder shooting.
applies to the 23 available AF points. Expansion for manually
912
, Zone AF (manual selection), or 23-