: Forums for software like vMix often have threads where users thank each other for a "useful post" regarding video codecs or MP4 recording settings.

Based on available information, does not appear to be a widely recognized viral video or a standard technical term. The filename structure and the "useful post" tag are common in niche communities (like Discord, Telegram, or private forums) where files are shared for specific tutorials, meme templates, or software assets.

: "tsunl" might be an abbreviation for a specific game, software, or user (e.g., "Tsun" is a common prefix in anime-related gaming communities). The "_9" would indicate it is the ninth in a series of resources.

: Bots on Reddit often generate links to .mp4 files and users frequently comment "this is a useful post" in response to helpful automated links.

Video Input Best Practices - vMix Forums

: Files with these names are often shared in video editing communities (like After Effects or CapCut groups) as "useful" overlay or transition presets. Common Contexts for ".mp4" and "Useful Post" References

If you are seeing this on a social platform or forum, it likely refers to one of the following: