Standard butter (minimum 80% milkfat) classifies under 040510. Anhydrous milkfat and butteroil — which have had virtually all water and milk solids removed, typically reaching 99%+ milkfat — classify under 040590 as other milk-derived fats and oils. The distinction matters because the two subheadings may carry different duty rates and TRQ treatment, and AMF is often used as an industrial ingredient rather than a retail product.

Dairy spreads are products made from milk fats that are formulated to be spreadable at refrigerator temperatures, often with a milkfat content below the 80% threshold required for butter. They classify under 040520 rather than 040510. If a product is labeled or marketed as butter but does not meet the US standard of identity (minimum 80% milkfat), it may be refused entry or required to be relabeled, so compositional documentation from the foreign manufacturer is important.