Canon MARK IV User Manual (en)

Custom Function Settings
C.Fn III -13     AF point display during focus
0: On
1: Off
The AF point will not light in red other than during AF point selection.
2: On (when focus achieved)
During AF, the manually-selected AF point will no longer light faintly. 
After AF starts, the AF point will light only when it achieves focus.
C.Fn III -14     AF point brightness
0: Normal
1: Brighter
Makes the AF point light up brighter in red.
C.Fn III -15     AF-assist beam firing
Enables or disables the EOS-dedicated Speedlite’s AF-assist beam.
0: Enable
The external Speedlite will emit the AF-assist beam when necessary.
1: Disable
2: IR AF assist beam only
Among external Speedlites, only those which have an infrared AF-assist 
beam will be able to emit the beam. Set this if you do not want the AF 
assist to be emitted as small flashes.
The external Speedlite’s Custom Function [AF-assist beam firing] set to 
[Disabled] will override this Custom Function’s 0 and 2 settings. The AF-
assist beam will not be emitted.