3903.90 - Other:

This category, "Other," within the broader classification of "Polymers of styrene, in primary forms," encompasses styrene-based copolymers and polymers that do not fall under the more specific classifications of polystyrene, styrene-acrylonitrile (SAN) copolymers, or acrylonitrile-butadiene-styrene (ABS) copolymers. It serves as a residual category for diverse styrene derivatives in their basic, unmanufactured state, such as powders, granules, or bulk liquids.

It is crucial to distinguish this category from its siblings. Unlike the direct listing of "Polystyrene" (which would likely encompass homopolymers of styrene), this "Other" category specifically includes copolymers where styrene is a component, but the resulting polymer's defining characteristics do not align with SAN or ABS. For instance, it would cover copolymers where styrene is combined with other monomers in proportions or combinations not designated for the more specific sibling categories.

The subcategories under 3903.90 further refine the classification of these "Other" styrene polymers. The "Methyl methacrylate-butadiene-styrene (MBS) copolymers" subheading provides a distinct place for this specific type of impact modifier, differentiating it from other styrene-based copolymers. The remaining "Other" subheading captures any remaining styrene polymers in primary forms that do not meet the criteria for any of the preceding subheadings within 3903.90, ensuring a comprehensive classification of all relevant styrene-derived materials.