MKV
Also known as: Matroska, .mkv
An open container format that holds almost any codec and any number of tracks, common for archives and awkward for delivery.
Updated
What it means
MKV is a container with very few restrictions. Any codec, any number of audio and subtitle tracks, chapters and attachments, which makes it the natural choice for archiving something complete and keeping it that way for years.
That flexibility is exactly what makes it awkward. A file that can hold anything tells a player nothing about whether this particular one will open, so Codec support becomes the only real question.
It is the open counterpart to MP4. One is standardised and universally supported, the other is capable and supported wherever somebody decided to support it.
How it works in practice
Remuxing is usually enough. Moving streams from MKV into an MP4 without re-encoding is a Remuxing operation, which takes moments and loses nothing at all in the process.
It is the right choice for keeping things. Several audio languages, subtitle tracks and chapters survive in one file, where an MP4 would need compromises or a set of separate files alongside it.
Editing applications vary. Some open it directly, some refuse outright, and some manage the video and ignore an audio track. Converting on import removes a question that otherwise returns at the worst possible moment.
What to watch out for
Browsers largely do not play it. A link to an MKV is a download rather than a video, which makes it the wrong format for anything delivered over the web.
Phones and televisions are inconsistent. Device compatibility with MKV depends on the player application rather than the hardware, so the same file behaves differently on two devices that otherwise look identical.
The extension tells nobody anything. An MKV can hold an ordinary modern codec or something nobody has installed, and a file that opens on one machine and not another is normal rather than evidence of anything broken.
How RawClip handles it
Uploaded MKV files are read like any other container, since what matters is the codec inside rather than the wrapper around it. The delivered result is an MP4, because that is the one container that opens everywhere without a conversation about it.