IVOA Recommendation: TAPRegExt: a VOResource Schema Extension for   Describing TAP Services
  This document describes an XML encoding standard for metadata about services implementing the table access protocol TAP [TAP], referred to as TAPRegExt. Instance documents are part of the service's registry record or can be obtained from the service itself. They deliver information to both humans and software on the languages, output formats, and upload methods supported by the service, as well as data models implemented by the exposed tables, optional language features, and certain limits enforced by the service. 
Markus Demleitner, Patrick Dowler, Ray Plante, Guy Rixon and Mark   Taylor
astro-ph.IM cs.DB