Both fall under subheading 251990, but they differ in production method and application. Dead-burned (sintered) magnesia is produced by high-temperature sintering of magnesite and is used primarily as a refractory material. Fused magnesia is produced by electric arc melting and offers higher purity and density, used in electrical insulation and specialty refractories. Classification does not separate them at the HS4 level, but the distinction matters for end-use compliance and pricing.

Subheading 251910 covers natural magnesium carbonate (magnesite) in its mined, unprocessed or minimally processed state. Once magnesite is calcined, sintered, or fused — converting the carbonate to an oxide — it moves to subheading 251990. The chemical transformation from carbonate to oxide is the operative boundary between the two subheadings.