19 CFR § 10.3025 - Transit and transshipment.
(1) Undergoes further production or any other operation outside the territories of the Parties, other than unloading, reloading, or any other operation necessary to preserve the good in good condition or to transport the good to the territory of a Party; or
(b)Documentary evidence. An importer making a claim that a good is originating may be required to demonstrate, to CBP's satisfaction, that the conditions and requirements set forth in paragraph (a) of this section were met. An importer may demonstrate compliance with this section by submitting documentary evidence. Such evidence may include, but is not limited to, bills of lading, airway bills, packing lists, commercial invoices, receiving and inventory records, and customs entry and exit documents.