One or more interrelated documents employing this grammar is called a 'topic map'. The purpose of a topic map is to convey knowledge about resources through a superimposed layer, or map, of the resources. A topic map captures the subjects of which resources speak, and the relationships between subjects, in a way that is implementation-independent. The key concepts in topic maps are topics, associations, and occurrences. Examples of such subjects might be the play Hamlet, the playwright William Shakespeare, or the 'authorship' relationship.
Topics can have names.
They can also have occurrences, that is, information resources that are considered to be relevant in some way to their subject. Finally, topics can participate in relationships, called associations, in which they play roles as members. We will speak of these locations as occurrences of these subjects. Note that different occurrences may relate to their subject in very different ways, which we would like to distinguish. In-depth discussions, brief mentions, and illustrations may need to be distinguished in order to allow the users to find more quickly what they need.