19 CFR 10.172 : Customs Duties (§ 0.1 To Parts 400-599) — (§ 0.1 To Parts 135-140) — U.S. Customs And Border Protection, Department Of Homeland Security; Department Of The Treasury (§ 0.1 To Parts 135-140) — Articles Conditionally Free, Subject To A Reduced Rate, Etc. (§ 10.1 To § 10.3034) — General Provisions (§ 10.1 To § 10.183) — Generalized System Of Preferences (§ 10.171 To § 10.178A) — Claim for exemption from duty under the Generalized System of Preferences.

A claim for an exemption from duty on the ground that the Generalized System of Preferences applies shall be allowed by the port director only if he is satisfied that the requirements set forth in this section and §§ 10.173 through 10.178 have been met. If duty-free treatment is claimed at the time of entry, a written claim shall be filed on the entry document by placing the symbol “A” as a prefix to the subheading of the Harmonized Tariff Schedule of the United States for each article for which such treatment is claimed.