IVOA Identifiers Version 2.0
  An IVOA Identifier is a globally unique name for a resource within the Virtual Observatory. This name can be used to retrieve a unique description of the resource from an IVOA-compliant registry or to identify an entity like a dataset or a protocol without dereferencing the identifier. This document describes the syntax for IVOA Identifiers as well as how they are created. 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. 
Markus Demleitner, Raymond Plante, Tony Linde, Roy Williams, Keith   Noddle
astro-ph.IM