Cellular rubber contains a network of open or closed cells (foam rubber, sponge rubber) that give it compressibility and low density. Noncellular rubber is solid throughout. The distinction is structural and determines whether 400811/400819 or 400821/400829 applies, regardless of the rubber polymer type.

Sheet material sold in bulk or cut to non-specific dimensions for further fabrication generally remains under 4008. Once the material is cut, punched, or molded into a finished gasket or seal of a specific shape for a defined application, it typically classifies as an article of vulcanized rubber under a later heading. The degree of further working and whether the article has a dedicated end-use shape are the key factors.