If your query is about the format itself, is a multimedia container used to store:
: Typically encoded with H.264/H.265 and AAC. 876874076983263232 mp4
In the context of Undertale , particularly on PC versions, the game uses MP4 files for certain scripted sequences. If your query is about the format itself,
: By locating the game's local files and identifying the specific .mp4 associated with this "feature," users can swap it with their own video file (renamed to match the original) to have custom video play during that segment of the game. General MP4 Information General MP4 Information : Most modern operating systems
: Most modern operating systems open MP4s natively via Windows Media Player or QuickTime .
If you are trying to mod a specific game or app, let me know the or what you want the video to do . A summary of MP4 format's advantages and features - Coconut