Class-X capacitors are connected line-to-line and must fail safely (open circuit) to prevent fire; Class-Y capacitors are connected line-to-ground and must not fail short to prevent shock hazard. US and international electrical safety standards require that capacitors in these positions carry certified X or Y ratings from a recognized testing laboratory. Importers supplying these parts to equipment manufacturers should verify that certification documentation accompanies each shipment.

Polypropylene film offers lower dissipation factor and better high-frequency performance than polyester (PET) film, making it preferred for power factor correction and audio crossover circuits. Polyester film provides higher volumetric efficiency at lower cost, suitable for general-purpose filtering. Both types fall within this same SITC heading — the dielectric polymer type does not create a separate classification, though it is a key procurement specification.