If you run a static electric field analysis using SOLID231 elements, you are performing a “pure” electric conduction analysis. Thus, permittivity is not used and only resistivity is required. SOLID231 cannot be used to calculate a static electric field in a dielectric. The resistivity is specified using the MP command. If you run a transient or harmonic electric field analysis using SOLID231 elements, both the permittivity and the resistivity are required, because both the conductive and the capacitive damping effects are included in the analysis. The resistivity and relative permittivity are specified using the MP command. The free-space permittivity for the model’s unit system is specified using the EMUNIT command.