Susceptibility is a measure of a Test Specimen's inability to function in the presence of an interference signal.

Immunity is the measure of a Test Specimen's ability to function despite the presence of an interference signal.

The interference signal may be propagate through free space as a radiated signal or through a conductor as a conducted signal. Lower frequency signals are more likely to propagate along conductors. Higher frequency signals are more likely to propagate through free space. Since "higher" and "lower" are relative terms, a Test Specimen is usually evaluated for both Conducted Susceptibility and Radiated Susceptibility with overlap in the measurement bandwidth.