IVOA Recommendation: IVOA Identifiers Version 1.12
  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. 
Raymond Plante, Tony Linde, Roy Williams, Keith Noddle, the IVOA   Registry Working Group
astro-ph.IM