Canon EOS-1D X Mark III Advanced User's Manual page 611

Note
When the camera is set to record RAW images to one CFexpress card and JPEGs
or HEIFs to the other, specify which images to transfer in the [RAW+JPEG
transfer] or [RAW+HEIF transfer] setting. Similarly, specify your transfer
preference when RAW+JPEG images or RAW+HEIF images are simultaneously
recorded to a single card.
Captured images are also stored on the card.
When images of the same size are recorded to both cards simultaneously, images
recorded to the card selected for [ Record/play] or [ Play] in [ : Record
func+card/folder sel.] are given priority for transfer.
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