Canon MARK IV User Manual (en)

Custom Function Settings
C.Fn III -19     Limit continuous shot count
Disable: Continuous shooting is not limited to any number of shots. 
Continuous shooting will be possible up to the current maximum 
burst displayed.
Enable: The continuous shooting will be limited to the number of shots set 
with [Register] after which the shooting will stop automatically.
Register: The continuous shot count can be limited between 2 to 99. After 
entering the settings, select [Apply].
C.Fn IV: Operation/Others
C.Fn IV -1     Shutter button/AF-ON button
0: Metering + AF start
1: Metering + AF start/AF stop
Pressing the <p> button during AF will stop the AF operation.
2: Metering start/Meter + AF start
This is useful for subjects which keep moving and stopping repeatedly. 
In the AI Servo AF mode, you can press the <p> button to 
repeatedly start or stop the AI Servo AF operation. The exposure is set 
at the moment the picture is taken. Thus, the optimum focusing and 
exposure will always be achieved as you wait for the decisive moment.
3: AE lock/Metering + AF start
Convenient when you want to focus and meter at different parts of the 
picture. Press the <p> button to meter and autofocus, and press 
the shutter button halfway to attain AE lock.
4: Metering + AF start/Disable
The <p> button will not function.
C.Fn IV -2     AF-ON/AE lock button switch
0: Disable
1: Enable
The functions of the <p> and <A/y> buttons will be switched 
between them.