IVOA Recommendation: Space-Time Coordinate Metadata for the Virtual   Observatory Version 1.33
  This document provides a complete design description of the Space-Time Coordinate (STC) metadata for the Virtual Observatory. It explains the various components, highlights some implementation considerations, presents a complete set of UML diagrams, and discusses the relation between STC and certain other parts of the Data Model. Two serializations are discussed: XML Schema (STC-X) and String (STC-S); the former is an integral part of this Recommendation. 
A. H. Rots
astro-ph.IM