Canon EOS 5D Mark III Instruction Manual (en)

 Setting the FlashN
You can set the flash-sync speed for 
flash photography in the aperture-priority 
AE (f) mode.
: Auto
The flash sync speed is set automatically within a range of 1/200 
sec. to 30 sec. to suit the scene’s brightness. High-speed sync is 
also possible.
: 1/200-1/60 sec. auto
Prevents a slow shutter speed from being set in low-light conditions. 
It is effective for preventing subject blur and camera shake. 
However, while the subject will be properly exposed with the flash, 
the background may come out dark.
: 1/200 sec. (fixed)
The flash-sync speed is fixed at 1/200 sec. This more effectively 
prevents subject blur and camera shake than with [1/200-1/60 sec. 
]. However, in low light, the subject’s background will come out 
darker than with [1/200-1/60 sec. auto].
[Flash sync. speed in Av mode]
If [1/200-1/60 sec. auto] or [1/200 sec. (fixed)] has been set, high-speed 
sync is not possible in the <f> mode.