Ports are treated differently depending on whether they are assigned to "good" conductors. Good conductors are defined according to the following rules.
Electric loss tangent - If the material loss is defined with an electric loss tangent, the loss tangent value must be at least 100.
Electrical conductivity - If the material loss is defined in terms of an electrical conductivity, the conductivity must be at least σmin, where σmin = 1000 S/m. The same rule applies for impedance boundary conditions that have been defined in terms of an electrical conductivity.
Surface resistivity - If an impedance boundary condition is defined in terms of a surface resistivity, the effective conductivity depends on frequency. A lossy material is considered a good conductor if the resistance is at most ωμ0/(2σmin), where ω is the radial frequency, μ0 is the permeability of free space, and σmin = 1000 S/m, as given above.
If you use a resistivity to define an impedance boundary, note that the boundary's conductivity will change with frequency. The boundary may be a good conductor at the AMR frequency, and a poor conductor at other simulation frequencies. Since lumped ports must connect two good conductors, the simulation will abort after the AMR process has completed in this case.