Canon EOS-1D X Mark III Advanced User's Manual page 546

Note
You can cancel the image transfer by selecting [Cancel] during the transfer.
You can select up to 999 files at a time.
With a Wi-Fi connection established, disabling the smartphone's power saving
function is recommended.
Selecting the reduced size for still photos applies to all still photos sent at that time.
Note that size still photos are not reduced.
When you use a battery to power the camera, make sure it is fully charged.
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