Identical and interchangeable materials: accounting segregation permitted to determine origin, subject to accounting standards, records and possible prior authorisation. Where identical and interchangeable originating, non-originating or undetermined materials are used, stocks must be physically segregated by origin; if segregation is impractical, a producer may use documented accounting segregation consistent with generally accepted accounting principles, provided it distinguishes origins and prevents overstating originating quantities. Producers using this method must only declare originating quantities, retain documentary evidence, provide stock-management information to authorities on request, and may be subject to prior authorisation.