Canon EOS 5D Mark III Instruction Manual (en)

 Customizing AF FunctionsN
You can set the AF operation characteristics and shutter-release timing 
during continuous shooting after the first shot with AI Servo AF.
: Equal priority
Equal priority is given to focusing and 
continuous shooting speed. In low light 
or with low-contrast subjects, shooting 
speed may slow down.
: Shooting speed 
Priority is given to the continuous shooting speed instead of achieving 
focus. The continuous shooting speed does not slow down. Convenient 
when you want to maintain the continuous shooting speed.
: Focus 
Priority is given to achieving focus instead of the continuous shooting 
speed. The picture is not taken until focus is achieved. Effective when 
you want to achieve focus before capturing the shot.
AI Servo 2nd image priority