Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 126)

3 Customizing AF Functions
126
You can set the AF operation characteristics and shutter-release timing
for One-Shot AF.
: Focus priority
The picture will not be taken until
focus is achieved. Useful when you
want to achieve focus before
capturing the image.
: Release priority
Priority is given to the timing of
shutter release instead of achieving
focus. This gives priority to capturing
the image rather than achieving
precise focus.
Note that the picture will be taken
even if focus has not been
achieved.
One-Shot AF release priority