19 CFR 10.138: Proof of use.

Within 3 years from the date of entry or withdrawal from warehouse for consumption, the importer shall submit in duplicate in support of his claim for free entry or for a reduced rate of duty a certificate executed by (1) the superintendent or manager of the manufacturing plant, or (2) the individual end-user or other person having knowledge of the actual use of the imported article. The certificate shall include a description of the processing in sufficient detail to show that the use contemplated by the law has actually taken place. A blanket certificate covering all purchases of a given type of merchandise from a particular importer during a given period, or all such purchases with specified exceptions, may be accepted for this purpose, provided the importer shall furnish a statement showing in detail, in such manner as to be readily identified with each entry, the merchandise which he sold to such manufacturer or end-user during such period.