The Plate interface is a specialization of the Shell interface, used for 2D modeling in the XY-plane. A plate model has its main action in bending out of the plane, but it can also treat in-plane forces. If the loads act only in the plane, using Solid Mechanics with the Plane Stress option is a better choice.
The in-plane stiffness of a shell is proportional to the thickness h, while the bending stiffness is proportional to
h3. The difference in stiffness along different directions can thus become very large. When an object is very thin, a shell model may be numerically ill-posed. It is then better to use the Membrane interface.