An important point to note here is the separation into two layers of the topics and their occurrences. This separation is one of the clues to the power of topic maps and we shall return to it later. Occurrences, as we have already seen, may be of any number of different types (we gave the examples of “monograph”, “article”, “illustration”, “mention” and “commentary” above). Such distinctions are supported in the standard by the concepts of occurrence role and occurrence role type.