I looked at out_stl_value in the mfx file and with the external flow box checked it is set to 1.0 which according to the manual 5.3.19, the domain outside of the STL geometry is excluded from computation, i.e., an internal flow is computed. So if external flow is checked it sounds like external is excluded. Am I correct? It is dependent on the facet normals which for my stl is pointing outward.