In the interior of the selection where a physics feature is active, its behavior is governed by its model equations and material properties. Boundary conditions apply to the geometric entities separating this region from the unspecified outside and sometimes also to interior entities of the same dimension (interface conditions). Therefore, boundary conditions on a 3D solid object apply to the exterior surfaces of the solid, and to interior surfaces embedded in the solid. On a shell geometry in 3D space, boundary conditions instead apply to the edges of the shell surface. In general, boundary conditions apply to geometric entities whose dimension is one less than the physics interface’s dimension.