The structure if a fully developed classification can be technically described in the following way:
(1) The whole collection (universe) of entities (in this case subject terms) may be divided into a series of fields...
(2) Each field may be divided into a series of facets...
(3) Each facet may be structured into a hierarchy, subdivided stage by stage into a series of characteristics...
(4) Rules may be provided for combining (coordinating) terms...
(5) Each field, facet and term may be coded to fix its position in the whole system...
(6)An alphabetical index to the terms, facets and fields may be provided, showing the code of each

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