: A paper ID like 2106.076... might be what you're recalling.
: The official ISO page for MPEG-4 Part 14 provides the technical specifications for the MP4 file format. 21676mp4
If the term refers to the MP4 container or a specific encoding, you might be looking for papers on the standard. : A paper ID like 2106
The number 21676 appears in various contexts that may have associated documentation: 21676mp4