4.1. Use in URI Protocol Slots

Because a URN is, syntactically, a URI under the "urn" scheme, in theory a URN can be placed in any protocol slot that allows for a URI (to name just a few, the "href" and "src" attributes in HTML, the base element in HTML, the "xml:base" attribute in XML, and the "xmlns" attribute in XML for XML namespace names).

However, this does not imply that, semantically, it always makes sense in practice to place a URN in a given URI protocol slot; in particular, because a URN might not specify the location of a resource or even point indirectly to one, it might not be appropriate to place a URN in a URI protocol slot that points to a resource (e.g., the aforementioned "href" and "src" attributes).

Ultimately, guidelines regarding when it is appropriate to use URIs under the "urn" scheme (or any other scheme) are the responsibility of specifications for individual URI protocol slots. This specification cannot possibly anticipate all of the relevant cases, and it is not the place of this specification to require or restrict usage for individual protocol slots.