Canon EOS-1D X Mark III Advanced User's Manual page 649

Selecting [Update] displays any images that have been added.
With [Select] selected, you can select multiple images.
Selecting individual thumbnails selects each image separately.
Selecting thumbnails again clears the selection.
Selecting [Select all] marks all images on the browsing screen as
selected.
Selecting [Clear all] clears selection of all images.
Selecting [Cancel] returns to the browsing screen.
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