I think I understand the general idea of the term, but I don't know if it is precise enough, or if there is an established consensus about this. In particular, I'd like to know:
- If the author of a book gives an explanation about something (say, in a magazine interview or a letter), and that explanation is not in the book, is it still in-universe?
- What if, instead of a published interview, this explanation is given during a live event (book signing, for example)? Does it make it "officially part of the universe"?
- Same with the writer or director of a movie, for example. If something is explained in the commentary from the DVD or a making-of, does it count as in-universe?
As I see it, in-universe should mean the published version (e.g. the book you can buy in a bookstore or the movie you can see at the cinema) and not later material, but the consensus might be different.