That is all simple enough under NAFTA. But, under the subsequent free trade agreements, no one ever specified a coding system for the preference criteria. As far as I am concerned, the lack of a uniform coding methodology is a travesty. Nevertheless, U.S. Customs and Border Protection's instructions on the implementation of the various agreements require that the certifying party specify how the good came to be originating.
Apparently, Customs has found this to be a bit of a problem as well. I gather that based on the fact that Customs just issued a reminder to the trade that FTA claims need to specify the basis for the origin determination. In that message, Customs also provided examples. Here is the text of the message free of copyright and in glorious courier font: