The region where the sub-model applies (the topo-object) can be a subdomain, a boundary set (in which case the model applies to this region of the boundary), or an internal region of the mesh, referred to as a PMesh. PMeshes are described in PMeshes.

You can define a sub-model on a group of elements, provided that they are all of the same physical dimension (0D, 1D, 2D, or 3D). Depending on the dimension of the domain of the sub-model, Ansys Polyflow will apply the appropriate contribution to the governing equations.