The foaf:primaryTopic property relates a document to the main thing that the document is about.

The foaf:primaryTopic property is functional: for any document it applies to, it can have at most one value. This is useful, as it allows for data merging. In many cases it may be difficult for third parties to determine the primary topic of a document, but in a useful number of cases (eg. descriptions of movies, restaurants, politicians, ...) it should be reasonably obvious. Documents are very often the most authoritative source of information about their own primary topics, although this cannot be guaranteed since documents cannot be assumed to be accurate, honest etc.

It is an inverse of the foaf:isPrimaryTopicOf property, which relates a thing to a document primarily about that thing. The choice between these two properties is purely pragmatic. When describing documents, we use foaf:primaryTopic former to point to the things they're about. When describing things (people etc.), it is useful to be able to directly cite documents which have those things as their main topic - so we use foaf:isPrimaryTopicOf. In this way, Web sites such as Wikipedia or NNDB can provide indirect identification for the things they have descriptions of.