The Exterior Field Calculation node is the feature that sets up the Kirchhoff-Helmholtz integral to perform the postprocessing of the radiated field under the high frequency assumption. It is per default added to all the source boundaries. The selection is not locked, so it can be modified but it should in general coincide with all the
Wall boundaries of the
Radiating Object. Only one
Exterior Field Calculation feature can be added to the interface.