The foaf:isPrimaryTopicOf property relates something to a document that is mainly about it.

The foaf:isPrimaryTopicOf property is inverse functional: for any document that is the value of this property, there is at most one thing in the world that is the primary topic of that document. This is useful, as it allows for data merging, as described in the documentation for its inverse, foaf:primaryTopic.

foaf:page is a super-property of foaf:isPrimaryTopicOf. The change of terminology between the two property names reflects the utility of 'primaryTopic' and its inverse when identifying things. Anything that has an isPrimaryTopicOf relation to some document X, also has a foaf:page relationship to it.

Note that foaf:homepage, is a sub-property of both foaf:page and foaf:isPrimarySubjectOf. The awkwardly named foaf:isPrimarySubjectOf is less specific, and can be used with any document that is primarily about the thing of interest (ie. not just on homepages).