Canon EOS-1DX Instruction Manual page 72

S Selecting the AF Area
AF Point Display Indications
Pressing the <S> button lights up the AF points that are cross-type AF
points for high-precision autofocusing. The blinking AF points are
horizontal-line sensitive. For details, see page 76.
AI Servo AF starting position
When the AF area selection mode is set to 61-point automatic selection
AF, you can manually select the AF point with which AI Servo AF is
started (p.103).
If [24: Initial AF pt,
pt], the AF point which was selected before the AF area selection mode
was switched to 61-point automatic selection can be used to start the AI
Servo AF.
With [25: Manual AF pt. selec. pattern], you can set either [Stops at AF
area edges] or [Continuous] (p.104).
72
AI Servo AF] is set to [Manual AF