Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 313)

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3 Setting the Movie Recording Quality
4K movie shooting
Shooting 4K movies requires a high-performance card. When
shooting H 8 / 7 movies, use a CFast card. For details on
card requirements for movie shooting, see “Cards that Can
Record Movies” on page 316.
Shooting 4K movies or movies at a high frame rate greatly
increases the processing load. Compared with normal movie
shooting, the camera’s internal temperature may increase faster
or may become higher. If the red
E icon appears during movie
shooting, it indicates that the card may be hot. Stop the movie
shooting and let the camera cool down before removing the card.
(Do not remove the card right away.)
From a 4K movie, you can select any desired frame to save it as
an approx. 8.8 megapixel (4096x2160) JPEG still image to the
card (p.374).
Movie-recording coverage
The respective area of the image sensor shown below is used for 4K
movies and Full HD movies.
To obtain better performance with the card, formatting the card with the
camera before shooting movies is recommended (p.74).
L Full HD
H 4K