This is an experimental namespace to map a subject to a Wikipedia page whose primary topic is that subject. The returned document serves two ways: the page itself is a Subject Indicator, and the URI + '#s' denotes the subject itself.
In RDF sense, the relation of them is expressed as
<> foaf:primaryTopic <#s> .
You can just use Subject URI to denote the subject directly, or use Subject Indicator URI as IFP for the subject. In XML TopicMamps, you may want to use Subject Indicator something like:
<topic id="XHTML"> <subjectIdentity> <subjectIndicatorRef xlink:href="http://www.kanzaki.com/ns/psi/en/XHTML"/> </subjectIdentity> </topic>