Canon EOS-1 D X Mark II Instruction Manual page 232

s: Shutter-Priority AE
Shutter Speed Display
The shutter speeds from "8000" to "4" indicate the denominator of the
fractional shutter speed. For example, "125" indicates 1/125 sec., "0"5"
indicates 0.5 sec. and "15"" is 15 sec.
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If the lowest f/number blinks, it indicates
underexposure.
Turn the <6> dial to set a slower shutter speed
until the aperture stops blinking or set a higher
ISO speed.
If the highest f/number blinks, it indicates
overexposure.
Turn the <6> dial to set a faster shutter speed
until the aperture stops blinking or set a lower ISO
speed.