A policy expression may be assigned a namespace, indicated either using the
@TargetNamespace attribute of the wsp:Policy element or inherited from a containing
element (e.g., wsdl:definitions, xs:schema). If no namespace is specified by a
container or by @TargetNamespace of the wsp:Policy element, the namespace is "".

A policy expression can also itself be a Web resource, hence identifiable by a URI. To
allow policy expressions to be embedded in arbitrary containing elements, the @wsu:Id
attribute may be used to indicate a fragment ID.