Both petroleum resins and polyterpenes are enumerated together under 391110, so the distinction does not affect the subheading. However, if a resin is derived from natural terpene sources and has been chemically modified to the point that it resembles a synthetic polymer, the chapter notes on natural versus synthetic polymers should be consulted to confirm it belongs in heading 3911 rather than heading 3913.

Polysulfones in primary forms are classified under 391190, which covers polysulfides, polysulfones, and other non-enumerated synthetic polymers. Importers of membrane-grade polysulfone should verify that the material is in a primary form (granules, powder, or solution) rather than already formed into a membrane, which would shift classification to a different chapter.