STC-X V1.20 is documented at http://www.ivoa.net/Documents/WD/STC/STC-20050225.html http://www.ivoa.net/Documents/WD/STC/STC-20050225.html, and the current version is documented at http://www.ivoa.net/Documents/latest/STC-X.html http://www.ivoa.net/Documents/latest/STC-X.html.
When an STC-X document is read using an XmlChan, the read operation produces an AST ObjectObject of the StcStc class, which is itself a subclass of RegionRegion. Specifically, each such Object will be an instance of StcSearchLocationStcSearchLocation, StcResourceProfileStcResourceProfile, StcCatalogEntryLocationStcCatalogEntryLocation or StcObsDataLocationStcObsDataLocation. See the description of the XmlChan class and the XmlFormatXmlFormat attribute for further details.