Canon EOS-1D X Mark III Advanced User's Manual page 203

Flash Sync Speed in Av Mode
You can set the flash-sync speed for flash photography in <
: Auto
The shutter speed is set automatically within a range of 1/250 sec. to 30 sec. to suit the
brightness. High-speed sync is also possible.
: 1/250-1/60sec. auto
Prevents a slow shutter speed from being set automatically in low-light conditions. It is
effective for preventing subject blur and camera shake. Light from the flash provides
standard exposure for subjects, but note that backgrounds may be dark.
: 1/250 sec. (fixed)
The shutter speed is fixed at 1/250 sec., which is more effective in preventing subject
blur and camera shake than with [1/250-1/60sec. auto]. However, in low light, the
subject's background will come out darker than with [1/250-1/60sec. auto].
Caution
High-speed sync is not available in <
auto] or [1/250 sec. (fixed)].
> mode when set to [1/250-1/60sec.
203
> mode.