One number for the entire file
MaxCLL is a single value covering everything from the first frame to the last. A display uses it to decide its tone-mapping strategy up front: if MaxCLL is 1,000 and the panel peaks at 600, it knows in advance that some compression is required and can apply a consistent curve rather than reacting frame by frame.
This is exactly the limitation that Dolby Vision and HDR10+ exist to fix. Their per-scene metadata lets the display treat a candlelit interior differently from a snowfield in the same film.
A specular highlight sets it
MaxCLL is driven by the brightest pixel, not the brightest scene. One frame containing a glint off chrome or a direct reflection of the sun can set MaxCLL for a whole clip, which means the figure often describes a few pixels rather than anything you notice while watching. That is intentional: it is a ceiling, not an average. MaxFALL is the average-facing counterpart.
Reading it
ffprobe -v error -select_streams v:0 \
-show_frames -read_intervals "%+#1" \
-show_entries frame_side_data=max_content,max_average \
-of default=nw=1 input.mp4
max_content is MaxCLL, max_average is MaxFALL. Both come from side data attached to the stream, so they appear on the frame rather than in the stream summary. A file can be valid HDR10 with these absent, and many displays cope, but a conforming file carries them.
Questions people ask
What happens if MaxCLL is missing?
Most displays cope. Without it a television falls back to its own analysis or a default assumption, which usually works and occasionally tone-maps more conservatively than needed. A file can be valid HDR10 with the field absent, but a conforming delivery carries it.
What is a typical MaxCLL value?
Content mastered at 1,000 nits commonly reports MaxCLL somewhere between 600 and 1,200, driven by whatever the single brightest specular in the whole file happens to be. A very high figure often reflects a few pixels of glint rather than anything you notice while watching.