According to § 3.13 of Leah F. Chanin, Specialized Legal Research (Smith KF240 .S69x), "[T]hese internal orders delegate Secretary of Treasury authority to enforce the Internal Revenue Code to the commissioner of Internal Revenue."
The U.S. Department of the Treasury website provides access to Treasury Orders since 1950. The orders are browseable by number and subject. The Treasury Department web page further notes that Treasury Orders "establish Treasury policy[] and establish the reporting relationships and supervision of officials."
