Industrial buyers typically specify Fisher Sub-Sieve Size (FSSS) particle diameter (commonly 0.6–20 µm) and minimum tungsten purity (often 99.9% or higher). Coarser powders are used in heavy-alloy and radiation-shielding applications, while ultrafine grades are preferred for submicron cemented carbide grades. These specifications should be documented on the commercial invoice to support classification and any end-use certificate requirements.

If ammonium paratungstate (APT) or tungsten oxide of Chinese origin is reduced to metal powder in a third country such as Canada or South Korea, US Customs applies a substantial-transformation analysis to determine origin. CBP has generally found that reduction of tungsten oxide to metal powder constitutes a substantial transformation, potentially conferring origin on the processing country — but this is a fact-specific determination, and importers should seek a binding ruling if the supply chain involves Chinese intermediate materials.