Canon EOS-1D X Mark III Advanced User's Manual page 298

Shooting Settings
Manually setting the ISO speed to 100–320 will make the dynamic range narrower.
Canon Log offers a dynamic range of approx. 800% at ISO 400 or higher.
ISO 100, 125, 160, 200, 250, and 320 are expanded ISO speeds. [ ] is displayed when
an ISO speed is set.
298