Subject, Classification by

a method of analytic and synthetic processing of printed material according to content. Classification by subject has many aspects and involves the selection of subjects and their designation, generally in standard terminology, as well as their characteristics and relationships. The word “subject” means any object of thought that is of interest to a reader and that is referred to in a text. The object of classification by subject is generally an entire published work—a book, brochure, or journal, or its parts—chapters, sections, pages, or articles.

Material that undergoes classification by subject may also be other sources of information or inquiries for information. In analyzing a text and formulating subject headings, the circumstances under which this classification method is used and the characteristics of the document being classified are taken into account. For example, when a book is classified by subject for a library catalog, consideration is given not only to its content but also to its form and purpose: whether it is meant for the reading public or for some special purpose.

The practical procedures and rules for classification by subject are the content both of the general methodology of this type of classification—used for all types of documents regardless of their subject matter—and of particular methodologies that are determined by the nature of the object being classified and by concrete information retrieval tasks; examples of such tasks are classification by subject of medical literature or of dissertation abstracts.

Classification by subject is used extensively in compiling subject catalogs, auxiliary subject indexes, subject indexes for literature, and other information retrieval systems. The common features of subject-heading languages and descriptory languages that have been established by information science permit classification by subject to be regarded as a form of indexing.