Canon EOS-1D X Mark III Advanced User's Manual page 491

Rating by Specifying the Range
While looking at the images in the index display, you can specify the first and last images for
a range to rate all the specified images at once.
1.
Select [Select range].
2.
Specify the range of images.
3.
Press the <
Select the first image (start point).
Next, select the last image (end point).
A checkmark [ ] will be appended to all the images within the range
between first and last images.
> button.
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