2621 - Other slag and ash, including seaweed ash (kelp); ash and residues from the incineration of municipal waste:

HTS code 2621 encompasses a diverse range of inorganic residual materials, specifically "other slag and ash, including seaweed ash (kelp); ash and residues from the incineration of municipal waste." This category serves as a residual provision for inorganic byproducts not specifically classified elsewhere within Chapter 26 or other chapters. It is crucial to note that this classification applies to materials that have already undergone a process of formation, such as smelting or combustion, and are not raw ores.

This category is distinct from earlier HTS chapters which deal with live animals. While Section V, "Mineral Products," broadly covers mineral-based substances, code 2621 specifically addresses waste and byproduct materials, differentiating it from raw ores (Chapter 26) or other manufactured mineral products found in different sections. It is important to distinguish this category from industrial slags covered in other headings of Chapter 26, focusing here on the specific types of ash and residues mentioned in its description.

Within HTS 2621, two subcategories provide further granularity. Subheading 2621.10.00.00 specifically covers "Ash and residues from the incineration of municipal waste," isolating materials derived from the combustion of household and commercial refuse. The remaining materials, not fitting the description of municipal incineration ash or residues, are classified under the residual subheading 2621.90.00.00, which includes items such as seaweed ash (kelp) and other slag and ash not specifically accounted for.