When post-processing stresses on surface bodies, several options are available. – For stresses derived from the stress tensor (components of the stress tensor, Von Mises stress, principal stress, intensity), you can choose the position where you want to look at the stresses. By default, the “position” option is set to “top/bottom” : the stress plot varies linearly through the thickness from the top face to the bottom face. When setting the “position” option either to top, bottom or middle, you are specifically asking for the results obtained on the top face, bottom face, or middle surface. – For stresses specific to shell elements (bending and membrane stress), the result is obtained through a through-thickness integration, so there is no “position” option available.