In the code generation process an XML Schema is transformed to an
    EMF Ecore model, which is then used as a meta model by EMF. XSD complex
    data types are mapped to EClasses, XSD simple data types are mapped to EMF
    data types defined in 
    org.eclipse.emf.ecore.xml.type.XMLTypePackage  and 
    org.eclipse.xtend.typesystem.xsd.XSDMetaModel  maps them to Xpand
    data types. The document 
XML
    Schema to Ecore Mapping
 explains the mapping's details.
    http://www.eclipse.org/modeling/emf/docs/overviews/XMLSchemaToEcoreMapping.pdf