Specifies dimensions that will ensure mechanical interchangeability of rear-mounted plug-In units (Transition Modules) in sub-racks based on American National Standard for Mechanical Core Specifications for Microcomputers Using IEC 603-2 Connectors, ANSI/IEEE 1101.1-1992; American National Standard for Additional Mechanical Specifications for Microcomputers Using the IEEE 1101.1 Equipment Practice, ANSI 1101.10-1996; IEC 297-3; and IEC 297-4.