The primary purpose of classification is to facilitate the retrieval of technical subject matter. In order to reliably retrieve technical subject matter using the CPC classification system it is important for all technical subject matter to be consistently classified.
CPC has its own classification rules which appear in the scheme, definitions, or both. See MPEP § 905.02. In the absence of any specified classification rules in the scheme and definitions, classification practice follows the rules used in the IPC, as set forth in the IPC Guide. The IPC Guide is available at www.wipo.int/classifications/ipc/en/.