Canon EOS-1D X Mark III Advanced User's Manual page 861

AF area selection mode (
Pressing the <
mode.
To continue using the current AF point when you press the <
set [AF start position] to [Manually selected AF point], and to continue using the current
AI Servo AF characteristics, AF operation, and AF area selection mode, select [Maintain
current setting].
Note
You can register separate AF points for vertical (grip up or down) and horizontal
shooting when [
Area+pt] or [Separate AF pts: Pt only].
[Registered AF point] takes precedence if both [Registered AF point] and [AF
area selection mode] are set in [AF start position].
)
> or < > button starts AF in your specified AF area selection
: Orientation linked AF point] is set to [Separate AF pts:
861
> or < > button,