I have already read all the design condition in the Design tool from the user guide, but I don't know exactly which one I should use for my problem. Can you help me further? Let me explain the problem further. According to the figure below, I have a 3D flat plate:

This geometry is supposed to be optimized with adjoint solver. During optimization, by changing the shape of the geometry, the thickness is reduced in some areas and increased in some areas. I have a design condition, and that is that the thickness of the geometry should remain constant during optimization, and in fact, the upper and lower surfaces of the geometry should be deformed at the same time, so that the distance between the two surface remains constant and unchanged.

Did you understand what I mean?