See screenshots. Entire model is composed of surfaces only.
Both surfaces referenced are set to "bottom" for shell offset. When I render the thickness in the meshed view it looks fine (material is below surface). But when I either right-click "edit definition" or I use the shell manager the surface of the one towards the back shows above the surface and the one in the front shows below the surface. So they're not consistent.
And which is the right/correct offset version that gives accurate results?
Also, I've never figured out what "bottom surface" and "top surface" is in reference. Is it the coordinate system (positive and negative directions)? The "outside" or "inside" of the part somehow? In reference to other adjacent geometry? The manual doesn't make this clear.
3-d preview. Orange is supposed to be the bottom face color
Fwd surface from "edit definition". Green is supposed to be top surface color (but it's on bottom)
Aft surface from "edit definition". Green is supposed to be the top surface color (but it's not even on the surface)
Shell manager with "preview offset" selected. At least consistent with "edit definition" view but definitely doesn't match meshed view.