Yes, a meta data note in your system that corresponds to the file that are not obvious from the context - either because they may have some inherent ambiguity in the matter, or because they might be hard to find – I have some files split across different drives because different clients can access different systems, being on loan is, of course the classic library example.
another case, I’m thinking of are resource libraries for instance - not just ebooks, but audio and video sources, these are likely to be in dedicated apps, or in a suite of apps given the reference manager can’t hold it all, in which case there is just a category that points to the app, as it were, or where they are in different places in different forms. But that’s another story, I think. Do you have a thread on that already?