27 C.F.R. § 19.257
The proprietor will incur a tax liability greater than the internal revenue tax imposed by 26 U.S.C. 5001(a)(1) , if spirits originally imported for nonbeverage purposes are transferred from customs custody to TTB bonded premises pursuant to 26 U.S.C. 5232 , and the proprietor subsequently decides to withdraw the spirits for beverage purposes. If the spirits would have been subject to a higher duty had they been imported for beverage purpose, the proprietor must pay a tax equal to the difference between the higher duty and the duty actually paid. Proprietors will refer to this additional tax as "additional tax-less duty" and pay it at the same time and in the same manner as the distilled spirits excise tax. Proprietors must compute the amount of "additional tax-less duty" owed by applying this rate to the total quantity of proof gallons withdrawn. The proprietor must make a separate entry on the tax return labeled "additional tax-less duty" and show the amount of tax due.
27 C.F.R. §19.257
26 U.S.C. 5001