Canon EOS-1D X Instruction Manual page 349

AI Servo AF characteristics (p.85)
Press the <p> or <A> button to perform AF with [Case1] to
[Case6].
AF mode (p.66)
Press the <p> or <A> button to perform AF with AF mode
set.
Super high speed continuous shooting (p.116)
Press the <p> or <A> button to perform <K> Super high-
speed continuous shooting when <o> High-speed continuous
shooting is set.
If you want to use the AF point selected when you press the <p>
or <A> button, set [AF start point] to [Manually selected AF point]. If
you want to instead retain the AI Servo AF characteristics, AF mode,
and drive mode that have been set, select [-].
85: Custom Controls
349