Ballot on URIs and Namespaces
Ballot on relative URI References in namespace declarations (3-17 July 2000).
Namespace Versioning
Major changes in the schema of the namespace would result in a new version of this namespace. So the namespace URI needs to specify a version.
Namespace URIs
RFC 3986 - Uniform Resource Identifier (URI): Generic SyntaxEach URI begins with a scheme name that refers to a specification for assigning identifiers within that scheme.
Why URI?

The only requirement for a namespace definition is to be unique within all the applications, it possibly can be used in. There is no need to specify a URI scheme, or some www... subdomain.

Most contexts about XML talk about "namespace URIs" and most publicly available namespaces are either http://... URLs or URNs.

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