The scope of a specialised documentary classification is usually designated by its title, the subject field that it professes to classify. It is no easy task to state what is meant by a subject field. In general it can be expressed as Thing-Activity. A definable group of things... is selected, and from the many relations in which they subsist a certain number are selected as relevant.

p. 11. - Classification and indexing in science (1958) - Chapter 1: The need for classification