Canon MARK IV User Manual (en)

Custom Function Settings
C.Fn I -7     Spot metering link to AF point
0: Disable (use center AF point)
1: Enable (use active AF point)
If [8C.Fn III -10: Selectable AF point] (p.223) is set to [1: 19 points], 
[2: 11points], [3: Inner 9 points] or [4: Outer 9 points], spot metering 
linked to the selected AF point is possible. With automatic AF point 
selection, spot metering will be at the viewfinder center.
C.Fn I -8     Safety shift
0: Disable
1: Enable (Tv/Av)
This works in the shutter-priority AE (s) and aperture-priority AE (f) 
modes. When the subject’s brightness changes erratically and the 
correct autoexposure cannot be obtained, the camera will change the 
exposure setting automatically to obtain a correct exposure.
2:  Enable (ISO speed)
This works in the Program AE (d), shutter-priority AE (s), and 
aperture-priority AE (f) modes. When the subject’s brightness 
changes erratically and the correct autoexposure cannot be obtained, 
the camera will change the ISO speed automatically to obtain a correct 
C.Fn I -9     Select usable shooting modes
Disable: All the shooting modes (a, s, f, d, BULB) will be 
Enable: Only the shooting modes set with [Register] will be selectable.
Register: To make a shooting mode unselectable, uncheck the checkmark 
<X>. After entering the settings, select [Apply].
If C.Fn III -10 is set to [0: 45 points], spot metering at the center will be set 
even if C.Fn I -7 is set to [1: Enable (use active AF point)].
Even if the exposure settings settable range has been changed with 
C.Fn I -3,12,13, the safety shift will override it when necessary to obtain 
the correct exposure.
With settings 1 and 2, safety shift also works with flash.