A retrieval system can be studied at three levels: - Brian Campbell Vickery

In scientific information, then, we find that subjects - the themes and topics on which books and articles are written - cluster into fields, each of which can be analysed into its characteristic set of facets of terms.

The most important characteristic of documentary classification is that it is concerned with subjects, not just entities of taxonomic classification. What is the nature of the specific subjects - the themes on which books, parts of books, articles or parts of articles are written? A study of book titles alone would suggest that literary subjects have simple names like 'War, Religion', 'Boats', 'Musica; pitch', 'Colour', 'Acridines', 'Wild flowers', and so on. But the study of articles on the documentation level reveals that such titles are simple in appearance only. Such a literary subject is in reality a complex aggregate of specific subjects, eahc which is the main theme discussed from one particular aspect.

The structural tools developed by the profession – hierarchical classifications, facets, thesaural relations, topic maps, the predicates used in ontology – will still be of use, provided that they can in each case pass a test of utility.