4.2 Persistent Identifiers
4.2.1 Even before the issue of digitisation made it critical, libraries, archives and audio collections generally have tended to develop systems with varying degrees of sophistication, which allow them to access their materials. These numbering systems, which tend to be unique within their own domain, can be incorporated into more universal naming schemes with the addition of a unique name for the domain or institution. This kind of structure allows maximum flexibility to an organisation in the local identification of its resources, whilst allowing the identifiers to be incorporated into a global system with the addition of an appropriate naming authority component. These persistent identifiers are for the user of the content to be able to identify a work (as opposed to a file) which remains constant through time as a reference for that work regardless of how the file naming conventions have changed.
4.2.2 A Persistent Identifier (PID) is an identifier constructed and implemented such that the identified resource will remain the same independently of the location of its representation and independent of the fact that several copies are available at various locations. It means that the PIDs are URNs.