Primarily due to confusion, customers will request registration on items that cannot be registered, like pipe (which is not a fitting but a calculated part of a pressure vessel or piping system), or items that do not require registration for the service that they are in. It can be easier for a customer to request a CRN than determine if it is required or not. A manufacturer often cannot determine if a fitting needs a CRN without knowing their customer’s intended service.
A CRN is often used as a marketing tool. Competition with a registered competitor can lead to CRN registration even if it will never be required for the service the fitting will see. It is also less confusing to have a CRN even if it is not required.