Label: Identifier – An unambiguous reference to the resource within a given context. Recommended best practice is to identify the resource by means of a string or number conforming to a formal identification system. Examples of 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).

Examples:

Identifier=”http://purl.oclc.org/metadata/dublin_core/& quot;
Identifier=”ISBN:0385424728″