Usually, two different right-hand side contributions (or loads) must be defined for each step. The first step needs a stationary value for the contribution, and the second step needs the perturbed value for the contribution. The definition of these contributions differs between exclusive and contributing nodes, and this relates to the Harmonic Perturbation node, which can be added to a wide variety of physics interface nodes (for example, the Electric Potential and Electric Ground nodes for the Electric Currents interface).