Recommended practice is to identify the related resource by means of a URI. If this is not possible or feasible, a string conforming to a formal identification system may be provided. (en)
A [second property](/specifications/dublin-core/dcmi-terms/#http://purl.org/dc/terms/relation) 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](/specifications/dublin-core/dcmi-terms/) for an explanation. (en)