Canon EOS-1D X Mark III Advanced User's Manual page 108

Caution
If the lowest aperture value blinks, it indicates underexposure.
Turn the <
blinking or set a higher ISO speed.
If the highest aperture value blinks, it indicates overexposure.
Turn the <
blinking, or set a lower ISO speed.
Note
Shutter speed display
Shutter speeds from 8000 to 4 indicate the denominator of the fractional shutter
speed. For example, 125 indicates 1/125 sec. Additionally, 0"5 indicates 0.5 sec.
and 15", 15 sec.
> dial to set a slower shutter speed until the aperture value stops
> dial to set a faster shutter speed until the aperture value stops
108