Canon EOS-1D X Mark III Advanced User's Manual page 379

Lens drive when AF impossible
You can specify lens operation that applies when autofocusing on a subject is not possible.
[ ] Continue focus search
If focus cannot be achieved with autofocus, the lens is driven to search for the precise
focus.
[ ] Stop focus search
If autofocus starts and the focus is far off or if focus cannot be achieved, the lens drive
will not be performed. This prevents the lens from becoming grossly out of focus due to
the focus search drive.
Caution
[Stop focus search] is recommended for super telephoto lenses or other lenses
that are driven over a large focus area, to avoid significant delay from focus search
drive if the lens becomes greatly out of focus.
379