19 CFR 10.133 - Conditions required to be met.
When the tariff classification of any article is controlled by its actual use in the United States, three conditions must be met in order to qualify for free entry or a lower rate of duty unless the language of the particular subheading of the Harmonized Tariff Schedule of the United States applicable to the merchandise specifies other conditions. The conditions are that:
(a) Such use is intended at the time of importation.
(b) The article is so used.
(c) Proof of use is furnished within 3 years after the date the article is entered or withdrawn from warehouse for consumption.