Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 342)

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General Movie Shooting Cautions
Recording and Image Quality
If you use a card with a slow writing speed, a five-level
indicator may appear on the right of the screen during
movie shooting. It indicates how much data has not
yet been written to the card (remaining capacity of the
internal buffer memory). The slower the card, the
faster the indicator will climb upward. If the indicator
becomes full, movie shooting will stop automatically.
If the card has a fast writing speed, the indicator will
either not appear or the level (if displayed) will hardly
go upward. First, shoot a few test movies to see if the card can write fast
enough.
If the indicator indicates that the card is full and movie shooting stops
automatically, the sound near the end of the movie may not be recorded
properly.
If the card’s writing speed is slow (due to fragmentation) and the indicator
appears, formatting the card may make the writing speed faster.
Indicator
Restrictions on MP4-format Movies
Note that generally the following restrictions apply to MP4-format movies.
Sound will not be recorded for approx. the last two frames.
When you play back movies on Windows, movie images and sound may
become slightly out of synchronization.