Canon EOS-1D X Mark III Advanced User's Manual page 739

Note
With [Priority:
inserted or removed.
Setting size via the rear LCD panel
1.
Press the <
2.
Select a card.
] set, the camera switches to the priority card when cards are
> button (
Turn the < > dial to select a card.
The card labeled with < > is used for recording and playback.
).
739