Surface bodies have a normal direction, identified by a green coloring when the surface body face is selected. Shell elements have a "top" surface (farthest in the positive normal direction) and a "bottom" (farthest in the negative normal direction).
By default, the shell section midsurface is aligned with the surface body, but you can use the Offset Type drop-down menu located in the Details view of a Surface Body object or an object scoped to a surface body to offset the shell section midsurface from the surface body:
Top - the top of the shell section is aligned with the surface body.
Middle (Membrane) (default) - the middle of the shell section is aligned with the surface body.
Bottom - the bottom of the shell section is aligned with the surface body.
User Defined - the user defines the amount of offset (Membrane Offset), measured in the positive normal direction from the middle of the shell section to the surface body (may be positive or negative value).
