Canon EOS-1D X Mark III Advanced User's Manual page 621

3.
Specify the caption.
Select [Caption], and on the screen displayed, select the content of the
caption.
4.
Select [Transfer].
The image is transferred with the caption. After transfer, display returns
to the [Image transfer] screen.
Caution
Other images cannot be selected from the [Transfer with caption] screen. To
select another image for transfer with a caption, view that image before following
these steps.
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