(Definition taken from the FERC Uniform System of Accounts, effective April 1, 1986.) "Retirement Units" means those items of plant which, when retired, with or without replacement, are accounted for by crediting the book cost thereof to the plant account in which included. ("Definitions" Item 32.) Each utility shall use list of retirement units as is in use by it at the effective date hereof or as may be prescribed by the Commission, with the option, however, of using smaller units, provided the utility's practice in this respect is consistent.