Fungible materials origin rules permit accounting segregation when physical segregation is impracticable, subject to GAAP safeguards. Rule 12 requires physical segregation by origin of identical and interchangeable fungible materials used in manufacture; where segregation causes significant cost or difficulty, an accounting segregation method may be used. That accounting method must be recorded and applied under generally accepted accounting principles of the State party, allow clear distinction between originating and non originating materials acquired, and guarantee that no more products receive originating status than would result from physical segregation.