Subheading 681511 covers carbon fibers in their raw, unworked filament or tow form — the precursor material before weaving or fabrication. Subheading 681512 covers woven or knitted fabrics made from those fibers for non-electrical uses. Subheading 681513 covers finished articles made from carbon fiber composites for non-electrical uses, such as structural panels or shaped components. The degree of processing — fiber, fabric, or finished article — determines the correct subheading.

Articles containing magnesite, periclase, dolomite, or chromite fall under 681591. These are typically refractory bricks, shapes, or linings used in high-temperature industrial furnaces and kilns. If such an article also contains asbestos or is primarily a friction material, it may instead classify under 6812 or 6813 — the specific composition and end use guide the determination.