The Surface Charge Density node provides the following surface-charge boundary condition for exterior boundaries (left) and interior boundaries (right):
Specify the surface charge density ρs at an outer boundary or at an interior boundary between two nonconducting media.
Add a contribution as a Harmonic Perturbation by right-clicking the parent node or click Harmonic Perturbation on the Physics toolbar. For more information, see Harmonic Perturbation — Exclusive and Contributing Nodes.