A knowledge of the fundamental concepts of EMI/RFI shielding will aid designers in selecting the gasket inherently best suited to a specific design.
All electromagnetic waves consist of two essential components, a magnetic field, and an electric field. These two fields are perpendicular to each other, and the direction of wave propagation is at right angles to the plane containing these two components. The relative magnitude between the magnetic (H) field and the electric (E) field depends upon how far away the wave is from its source, and on the nature of the generating source itself. The ratio of E to H is called the wave impedance, Zw.
This design guide goes over the basics of EMI/RFI Shields and Gaskets.