19 USC 1502 - Regulations for appraisement and classification

(a) Powers of Secretary of the Treasury 
The Secretary of the Treasury shall establish and promulgate such rules and regulations not inconsistent with the law (including regulations establishing procedures for the issuance of binding rulings prior to the entry of the merchandise concerned), and may disseminate such information as may be necessary to secure a just, impartial, and uniform appraisement of imported merchandise and the classification and assessment of duties thereon at the various ports of entry. The Secretary may direct any customs officer to go from one port of entry to another for the purpose of appraising or classifying or assisting in appraising or classifying merchandise imported at any port, and may direct any customs officer at any port to review entries of merchandise filed at any other port.
(b) Duties of customs officers 
It shall be the duty of all officers of the customs to execute and carry into effect all instructions of the Secretary of the Treasury relative to the execution of the revenue laws; and in case any difficulty arises as to the true construction or meaning of any part of the revenue laws, the decision of the Secretary shall be binding upon all officers of the customs.