Canon EOS-1D X Mark III Advanced User's Manual page 61

4.
Set the power switch to <
(3) Number of available shots
(4) Card selection icon
(5) Card 1
(6) Card 2
The top LCD panel shows how many shots are available.
The rear LCD panel shows which cards are loaded.
Cards labeled with < > are selected for recording.
Caution
Only CFexpress cards can be used.
Note
The number of shots available varies depending on remaining card capacity and
settings such as image quality and ISO speed.
Setting [ : Release shutter without card] to [Disable] will prevent you from
forgetting to insert a card (
> (
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