Canon EOS-1D X Mark III Advanced User's Manual page 376

One-Shot AF release prior.
You can set the AF operation characteristics and shutter-release timing for One-Shot AF.
[ ] Focus
The picture will not be taken until focus is achieved. Useful when you want to achieve
focus before capturing the image.
[ ] Release
Prioritizes shutter release over focus. Useful when capturing the decisive moment is
most important.
Note that the camera shoots whether or not the subject is in focus.
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