Generality and complexity of the paradigm may cause problems with right information interchange between data model and editors and end users. And let's assume we have to pass to somebody idea of what the documents are about and how are they interrelated. The model allows our information objects to be classified as occurrences of particular topics. In this way topic represents a metacategory for group of documents, but in the other way is completely defined by set of it's occurrences. Topics may be associated with other topics.
Association is expressed by a link.
Topics and associations may have assigned topic types and associations types respectively. Hierarchy classification means that we assign properties to objects in a tree-like structure. Humans have got used to tree-like hierarchies. We started to get experience with trees from Eden, but seriously, Aristotle is recognised as the father of hierarchical classification. Nowadays most of the information classification and presentation methods are based on tree paradigm.