In shell theory, all quantities refers to the midsurface. In the Shell interface, you can however use any boundary that is parallel to the midsurface for modeling. If the
reference surface where the mesh is located is not the midsurface, you can use an offset distance to describe the true location. If you have a full 3D geometry, there is thus no need to create the midsurface. You can place the mesh on either the top or bottom surface of the actual geometry.
Plates are similar to shells but act in a single plane and usually with only out-of-plane loads. The plate and shell elements in COMSOL Multiphysics
are based on the same formulation. The Plate interface for 2D models is a specialization of the Shell interface. In the following, the text fully describes the Shell interface, and the Plate interface is mentioned only where there are nontrivial differences.