Historicaly, any library, museum, archives services, etc have its own system and this is still a complex question. For the books, the isbn wins, but it doesn’t apply to old books; there are issn for serials, doi for articles, ark for archives, orcid, openid, idhal, or a lot of competitors to identify researchers. In my heritage projects, I prefer to use ark when I can, because it provides any number of free number easily. Anyway, most of the time there are multiple unique identifiers…