The 97% CaF₂ threshold separates metallurgical-grade fluorspar (≤97%, subheading 252921), used as a flux in steelmaking and cement, from acid-grade fluorspar (>97%, subheading 252922), which is the feedstock for hydrofluoric acid and downstream fluorochemicals. The distinction matters commercially because acid-grade commands a higher price and faces different end-use demand patterns.

Nepheline and nepheline syenite are grouped with leucite under subheading 252930, separate from feldspar (252910), because they are mineralogically distinct aluminosilicates with different alkali content and melting behavior. In ceramics and glass manufacturing, nepheline syenite is valued for its lower melting point and higher alumina content compared to feldspar, so the classification difference reflects a genuine commercial distinction.