Canon EOS-1D X Mark III Advanced User's Manual page 457

Specifying the Range of Images to Erase
While looking at the images in the index display, you can specify the first and last images for
a range to erase 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).
To select another image to be erased, repeat step 2.
> button.
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