Looking at several of the newsreader software packages that do have offline support, it looks like “subscription” is typically used to refer to that simple action of adding a newsgroup to a list of “favorite newsgroups” for quick access.

It sounds like Thunderbird is not an offline reader. Apparently it supports a limited form of marking a group for offline use, and that groups are explicitly marked for such download via a second mechanism, not subscription:

https://bugzilla.mozilla.org/show_bug.cgi?id=693668

Are the newsgroups set up for offline storage?

That would be the “select newsgroups for offline use…” button on attachment 566469 [details] ?

However, this does not download the bodies of posts in such a newsgroup, just the headers. These will include the title of the post, the author, and some other information about it, and I suppose might let you see a list of posts in a newsgroup, but won’t let you read posts in a newsgroup offline, which is normally how a newsreader software package with offline reading support works.

It sounds like the real problem is that we don’t automatically download bodies for offline retention in newsgroups, which I don’t think we’ve ever supported, but which I think is worthwhile to support.

Yes, “Select this newsgroup for offline use” might be misleading, as it does not mean that bodies will be saved as well when new headers are fetched.