Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 865)

AF area selection mode ( )
Pressing the < > or < > button starts AF in your specified AF area selection
mode.
To continue using the current AF point when you press the < > or < > button,
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 [
: Orientation linked AF point ] is set to [ Separate AF pts:
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 ].
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