Many ash and slag types have dedicated headings — granulated iron/steel slag, coal ash used as cement admixtures, and precious-metal-bearing residues each have their own classification with potentially different duty rates. The NES heading applies only after confirming the material does not fit a more specific category, making a thorough classification review essential before entry.

Seaweed ash was historically important as a source of iodine and potash before synthetic production methods became dominant. Today it represents a negligible share of trade under this heading; the commercial volume is driven by industrial by-product ashes from power generation and non-ferrous smelting rather than marine biomass combustion.