19 CFR § 10.542 - Third country transportation.

(a) General. A good will not be considered an originating good by reason of having undergone production that would enable the good to qualify as an originating good if subsequent to that production the good undergoes further production or any other operation outside the territories of the Parties, other than unloading, reloading, or any other process necessary to preserve the good in good condition or to transport the good to the territory of a Party.

(b) Documentary evidence. An importer making a claim that a good is originating may be required to demonstrate, to CBP's satisfaction, that no further production or subsequent operation, other than permitted under paragraph (a) of this section, occurred outside the territories of the Parties. 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.