The namespace declaration URI as a constant. The value is http://www.w3.org/xmlns/2000/, as defined in a backwards-incompatible erratum to the "Namespaces in XML" recommendation. Because that erratum postdated SAX2, SAX2 defaults to the original recommendation, and does not normally use this URI.
This is the Namespace URI that is optionally applied to xmlns and xmlns:* attributes, which are used to declare namespaces.