There is no global standard for the contents of NBNs; instead, they have a country-specific format.
NBNs are typically used for documents which do not have a publisher-assigned identifier such as an ISBN.
They can be used to identify media archived in national libraries, such as Ph.D. theses.
[1][2] For example, "urn:nbn:de:bvb:19-146642" is an NBN assigned in Germany ("de"), associated with Bavaria ("bvb" = Bibliotheksverbund Bayern), library number 19 (University Library Munich).
The NBN refers to a PhD thesis on heat detection in cattle.