Canon EOS-1D X Mark III Advanced User's Manual page 449

Specifying the Range of Images to Protect
While looking at the images in the index display, you can specify the first and last images for
a range to protect all the specified images at once.
1.
Select [Select range].
2.
Specify the range of images.
Select the first image (start point).
Next, select the last image (end point). The images in the specified
range will be protected and the <
To select another image to protect, repeat step 2.
> icon will appear.
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