19 CFR §181.21

1. (a)Declaration. In connection with a claim for preferential tariff treatment, or for the exemption from the merchandise processing fee, for a good under the NAFTA, the U.S. importer must make a formal declaration that the good qualifies for such treatment. The declaration may be made by including on the entry summary, or equivalent documentation, including electronic submissions, the symbol “CA” for a good of Canada, or the symbol “MX” for a good of Mexico, as a prefix to the subheading of the HTSUS under which each qualifying good is classified. Except as otherwise provided in 19 CFR 181.22 and except in the case of a good to which Appendix 6.B to Annex 300-B of the NAFTA applies (see also 19 CFR 102.25), the declaration must be based on a complete and properly executed original Certificate of Origin, or copy thereof, which is in the possession of the importer and which covers the good being imported.
2. (b)Corrected declaration. If, after making the declaration required under paragraph (a) of this section or under § 181.32(b)(2) of this part, the U.S. importer has reason to believe that a Certificate of Origin on which a declaration was based contains information that is not correct, the importer shall within 30 calendar days after the date of discovery of the error make a corrected declaration and pay any duties that may be due. A corrected declaration shall be effected by submission of a letter or other written statement to the CBP office where the original declaration was filed.