Certificate.
(6) Certificate. A certificate signed by the purchaser, or an officer or employee authorized by the purchaser to sign the certificate, may be accepted by a seller in support of a nontaxable sale to the purchaser. If it is impracticable to furnish a separate certificate for each sale because of the frequency of sales to such purchaser, a certificate covering all orders between given dates (such period not to exceed 12 calendar quarters) will be acceptable. The purchaser may revoke the certificate by sending a written revocation to the seller. The certificate and proper records of invoices, orders, etc., relating to sales made pursuant to such certificate, must be retained by the seller as provided in section 6001 and the regulations thereunder. The certificate shall be substantially in the following form: