A topic may be linked to one or more information resources that are deemed to be relevant to the topic in some way. Such resources are called occurrences of the topic. An occurrence could be a monograph devoted to a particular topic, for example, or an article about the topic in an encyclopaedia; it could be a picture or video depicting the topic, a simple mention of the topic in the context of something else, a commentary on the topic (if the topic were a law, say), or any of a host of other forms in which an information resource might have some relevance to the subject in question.