Canon EOS 1D Software Instruction Manual page 192

Personal Function Settings
Disables focus detection (search driving) by the lens drive
Normally, when you press the shutter button halfway and focus cannot be achieved, the lens
is driven to search the correct focus. This lens driving operation can be turned off.
When focus has been achieved once and refocusing fails, the subsequent lens driving
operation can result in grossly inaccurate focus. This subsequent lens driving operation is
turned off. P.Fn-14 is especially suited for super telephoto lenses.
If autofocusing is attempted while the lens is way out of focus, the lens may stop
operating. Focus manually until the subject outline becomes discernable, then
autofocus.
Disables the AF-assist beam from being emitted
This disables the external Speedlite's built-in AF-assist beam.
At a joint photo session, etc., this prevents the AF-assist beam's pattern projected on the
subject from being photographed by other photographers.
M-68