Canon EOS-1DX Instruction Manual page 342

3 Custom Function Settings
This function does not work with macro lenses whose actual aperture
f/number changes when the magnification changes.
This function does not work with movies.
If [Shutter speed] is set and the same exposure cannot be maintained
within the shutter speed range set with the [82: Set shutter speed
range], shutter speed will not be changed automatically.
If [ISO speed] is set and the same exposure cannot be maintained within
the ISO speed range set with the [ISO speed range], ISO speed will not
be changed automatically.
This function also works with changes for maximum f/number (smallest
aperture).
If you set [ISO speed] or [Shutter speed], perform 1, 2, or 3, and then
cancel 1, 2, or 3 so that the camera is back to its original state, the
original exposure setting will be restored without you manually changing
the ISO speed, shutter speed, or aperture.
If [ISO speed] is set to "L", "H1", or "H2", shutter speed may be changed
to maintain the same exposure.
342