Specifies dimensions which will ensure the mechanical and "Requirements and Tests of Environmental Conditions for Structures for Electronic Equipment" interchangeability of subracks and plug-in units, based on IEEE Std 1101.1, IEC 297-3, IEC 297-4, using IEC 603-2 based connectors and IEC 48D/1587-1 and IEC 48D/1587-3 requirements.