19 CFR § 181.64
General. This section sets forth the rules which apply for purposes of obtaining duty-free or reduced-duty treatment on goods returned after repair or alteration in Canada or Mexico as provided for in subheadings 9802.00.40 and 9802.00.50, HTSUS. Goods returned after having been repaired or altered in Mexico, whether or not pursuant to a warranty, and goods returned after having been repaired or altered in Canada pursuant to a warranty, are eligible for duty-free treatment, provided that the requirements of this section are met. Goods returned after having been repaired or altered in Canada other than pursuant to a warranty are subject to duty upon the value of the repairs or alterations using the applicable duty rate under the United States-Canada Free-Trade Agreement (see 10.301 of this chapter), provided that the requirements of this section are met. For purposes of this section, repairs or alterations means restoration, addition, renovation, redyeing, cleaning, resterilizing, or other treatment which does not destroy the essential characteristics of, or create a new or commercially different good from, the good exported from the United States.