26 U.S.C. § 5385

(a) Definition Specially sweetened natural wine is the product made by adding to natural wine of the winemaker’s own production a sufficient quantity of pure dry sugar, or juice or concentrated juice from the same kind of fruit, separately or in combination, to produce a finished product having a total solids content in excess of 17 percent by weight and an alcoholic content of not more than 14 percent by volume, and shall include extra sweet kosher wine and similarly heavily sweetened wines. (b) Cellar treatment Specially sweetened natural wines may be blended with each other, or with natural wine or heavy bodied blending wine in the further production of specially sweetened natural wine only, if the wines so blended are made from the same kind of fruit. Wines produced under this section may be cellar treated under the provisions of section 5382(a) and (c). Wine spirits may not be added to specially sweetened natural wine.