The critical distinction is functional: insulators (77323) are standalone devices designed to support and isolate conductors, while insulating fittings (77326) are components integrated into electrical machines or equipment — such as terminal supports, arc-chute plates, or bushing tubes. The SITC heading 77326 explicitly excludes insulators, so the end-use context and product design documentation are essential for correct classification.

High-alumina ceramics (90–99% Al₂O₃) are preferred where high dielectric strength and thermal stability are required, such as in arc-interruption components. Steatite and cordierite grades are used where lower thermal expansion or easier machinability is needed. Suppliers typically provide material certificates specifying alumina content, dielectric constant, and flexural strength, which procurement engineers use to match the fitting to the equipment's operating conditions.