An identifier is an association between a string (a sequence of characters) and an information resource. That association is made manifest by a record that binds the identifier string to a set of identifying resource characteristics.
National Bibliographic Agencies often act as the national maintenance agency for standard identifiers and take responsibility for their assignment. In other countries the role may be assumed by a trade, standards or commercial organisation.
Standard identifiers support interoperability throughout the supply chain. They are an essential component of resource description and an indicator that a resource has been formally published. National bibliographic agencies can use standard identifiers to locate additional metadata for resources and the following product identifiers are commonly used in national bibliographies:
- International Standard Book Number (ISBN) for print and electronic books
- International Standard Serial Number (ISSN) for print and electronic journals
It is strongly recommended that NBAs retain standard identifiers whenever associated with resources or recorded in accompanying metadata. In ISBD descriptions, the relevant standard identifier is entered in the standard number and terms of availability area of the bibliographic description.