  An IVOA Identifier is a globally unique name for a resource. This name can be
used to retrieve a unique description of the resource from an IVOA-compliant
registry. This document describes the syntax for IVOA identifiers as well as
how they are created. An IVOA identifier has two separable components that can
appear in two equivalent formats: an XML-tagged form and a URI-compliant form.
The syntax has been defined to encourage global-uniqueness naturally and to
maximize the freedom of resource providers to control the character content of
an identifier.
