Codec support
Also known as: decoder support, format support
Which compression formats a device or application can decode, and the most common reason a perfectly good file will not play.
Updated
What it means
Support is about the codec, not the wrapper. An MP4 is a box, and what matters is whether the thing inside it can be decoded by whatever is trying to play it. The extension on the end is not a promise about anything.
That distinction explains most of the confusion. Two files with the same extension can behave completely differently, because one holds H.264 and the other holds something considerably newer.
The list is short in practice. H.264 (AVC) everywhere, H.265 (HEVC) unevenly, AV1 increasingly, and anything else by arrangement rather than by default on a given device.
How it works in practice
H.264 is the compatibility floor. Anything that has to play for an unknown audience ends up there, however much more efficient the alternatives are. Reach beats efficiency whenever the audience is unknown.
Delivering two versions solves it. A modern codec for devices that support it and a safe one for everything else is what every streaming service already does, and the arrangement has not changed in a decade.
Converting on import removes the question from an edit. Transcoding once means the timeline never meets a codec the application dislikes halfway through a long project.
What to watch out for
An unsupported file produces a useless message. A player declines to open it and says almost nothing about why, which is a Playback error with the cause hidden. Guessing is the normal diagnostic, which is why this wastes so much time.
Hardware support is a separate question from software support. A device can decode a format slowly in software and smoothly with Hardware decoding, and only one of those two is actually watchable.
Professional codecs are not delivery codecs. ProRes and DNxHD play in editing applications and almost nowhere else, which is correct and still surprises people sending files to clients.
How RawClip handles it
Uploaded footage is decoded server side, so camera codecs that a browser or a phone cannot open are handled normally. The delivered result goes the other way and uses whichever widely supported codec is safest at the time, since it has to open for anybody.