Canon EOS-1D X Mark III Advanced User's Manual page 910

Group F
Not all 191 AF points can be used for autofocusing, but the 139 points shown here are
available. All the AF area selection modes are selectable. During automatic AF point
selection, the outer frame marking the AF area (Area AF frame) will be different from 191-
point Automatic selection AF.
: Cross-type AF point. Offers excellent subject tracking and high-precision focusing.
: AF points sensitive to horizontal lines.
: Disabled AF points (not displayed).
910