Explain discontinued number

A discontinued number is a classification number that is no longer used or recognized in a particular classification system. Discontinued numbers are typically used in library classification systems to represent subjects or topics that are no longer considered relevant or up-to-date, or that have been subsumed by other subjects or topics.

For example, in the Dewey Decimal Classification system, discontinued numbers are used to represent subjects that have been removed from the classification system or that have been replaced by new subjects. When a subject is discontinued, the classification numbers associated with that subject are no longer used or recognized, and materials that were previously classified using those numbers are reclassified using different numbers.

Discontinued numbers are an important tool for keeping classification systems current and relevant, and they help to ensure that materials are accurately and consistently classified in a way that reflects their subject matter and the current state of knowledge in a particular field.