Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 440)

5.
Save the image.
Select [ ] (1).
The save screen will appear.
To save it as a new file, select [ New file ], or to save it and overwrite the
original movie file, select [ Overwrite ].
On the confirmation screen, select [ OK ] to save the edited movie and
return to the movie playback screen.
Caution
Because editing is performed in approx. 1 sec. increments (at the position indicated
by [
] at the bottom of the screen), the actual position where movies are trimmed
may differ from your specified position.
Movies recorded with another camera and RAW movies cannot be edited with this
camera.
You cannot edit a movie when the camera is connected to a computer.
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