This is a value which can be used in the "Sutherland’s Handbook for Bicycle Mechanics" spoke calculation system. It uses the Effective Rim Diameter value to determine the Rim Correction Factor. This is useful for updating a Sutherland’s Manual with information about a new rim that may have been introduced after the current manual was printed. It is not needed when using this calculator as a stand-alone system and is offered only as a service to owners of the Sutherland's Manual.