A unique persistent identifier enables systems to permanently combine or separate information from different sources about the same learning resource. The persistent identifier used for research material and metadata is the Uniform Resource Name. URNs do not refer to a physical location of a file on a server, but to a name.
The National Bibliographic Number [NBN] is a persistent identifier that makes this possible. Specifically for URN:NBN:NL:UI, the emphasis is on identifying permanently accessible research results in the form of publications and datasets.