Imagine the case where a small air gap is in between two solid bodies, the solid bodies and the air gap are modeled as a multilayer shell conduction wall. The air gap acts as a heat resistance for the heat flow. In reality for high temperatures radiation through the air gap between the two solids might have a large impact on the transferred heat flow. This cannot be modelled using the multilayer shell conduction wall model. The air gap has to be modeled as a cell zone instead.