3907.61.00.10 - Having a viscosity number of 78 ml/g or more but not more than 88 ml/g

This HTS category, 3907.61.00.10, specifically classifies poly(ethylene terephthalate) (PET) resins that possess a defined viscosity range. These resins are characterized by having a viscosity number falling between 78 ml/g and 88 ml/g, inclusive. This precise viscosity measurement is a critical physical property that dictates the processing characteristics and ultimate applications of the PET resin.

As this category is standalone at its current level, there are no direct siblings to distinguish it from. Its placement within the broader context of polycarbonates, alkyd resins, polyallyl esters, and other polyesters, in primary forms, highlights that it is a specific type of polyester. The primary forms generally refer to powders, granules, flakes, or blocks prior to their further manufacturing into finished articles.

The only subcategory under this node is 3907.61.00.50, designated as "Other." This suggests that any poly(ethylene terephthalate) resin that does not meet the specific viscosity criteria of 78 ml/g to 88 ml/g will be classified under this "Other" category. Therefore, the distinction lies solely in the measured viscosity number, differentiating this specific grade of PET resin from other PET resins or indeed other types of polyesters in primary form.