A demo is a recording made to present a song at an earlier stage of its making, before the version that is released as the finished song. In the music industry the word is general and carries no fixed definition, so what a given demo contains depends on who made it and why. When "(DEMO ver.)" or "Demo" follows a title on an official upload, we read that as an ordinary word in loose use, not as a technical term with a rulebook behind it.
On an official upload, the label means the uploader is presenting this particular recording and calling it a demo. It is the uploader's own word. The upload usually does not say who made the demo, when it was made, or what differs from the finished version, so the label tells you the recording is presented as a demo and nothing more. Why an official channel posts one is also usually left unsaid. These are possibilities, not claims: to share a recording that exists alongside the finished one, to mark a project or an anniversary, or as part of a rollout of a release. None of them can be said to apply to a given upload.
A demo is easy to mix up with its neighbours, so here is the one-line contrast: a remix reworks the finished recording and credits a remixer, a re-recording is the act performing its own song again, a language version is the same act in another language, and a live version is a recording of a performance. A demo is labelled by how far along the recording was presented, not by who changed it. In practice it is a separate recording that appears as its own entry, with its own page and its own counts, so numbers on it are not comparable with the finished song's.
The honest limit is that the word is used loosely, so a demo label is the uploader's description of the recording and not a promise about its contents. Uploads that carry the same label might differ in who made them and in how far along each recording was. That is why the surrounding text matters. The description is where an explanation would appear, and an Official Audio mark or a set of credits would add detail. If the description is silent, the label is all the information there is.