grouping of concepts of the same inherent category
Examples of categories that may be used for grouping
concepts into facets are: activities, disciplines,
people, materials, living organisms, objects, places
and times. e.g.
(1) animals, mice, daffodils and bacteria could all be members of a living organisms facet;
(2) digging, writing and cooking could all be members of an activities facet;
(3) Paris, the United Kingdom and the Alps could all be members of a places facet.
Categories are normally chosen so that facets are mutually exclusive; a concept cannot then occur in more than one facet. In a classification scheme, facets may be restricted to a single discipline, such as a diseases facet in medicine, or may be common facets such as people, time, place and form, which apply across all disciplines. Facets may be subdivided into mutually exclusive subfacets.
Some writers use the term "facet" to specify the role that a concept plays in a complex concept, as well as the category to which it belongs. For example, they may say that materials can belong to "raw materials" or "products" facets, and people may be in "agents" or "patients" facets. For clarity, it is better to avoid this usage, keeping the term "facet" for fundamental categories such as "materials" or "people" and specifying roles separately. Both facets and roles are used in setting up rules for citation order.
Other writers use the term "facet" to mean "attributes" or "properties", confusing them with characteristics of division. There may be multiple characteristics of division of concepts within a single facet, e.g. within a materials facet there may be a concept of wines, subdivided into several arrays, not mutually exclusive, each headed by a node label such as , , , and so on. Any specific wine can be listed in several of these arrays. Searching by these is better called searching by parameters or characteristics rather than by facets.