Recommended best practice is to identify the resource by means
of a string or number conforming to a formal identification
system.
Example formal identification systems include the Uniform
Resource Identifier (URI) (including the Uniform Resource
Locator (URL)), the Digital Object Identifier (DOI) and the
International Standard Book Number (ISBN).
A second property with the same name as this property has been declared in the dcterms: namespace (http://purl.org/dc/terms/). See the Introduction to the document "DCMI Metadata Terms" (http://dublincore.org/documents/dcmi-terms/) for an explanation.