Creo Simulate uses the surface pairs that you define to form a network of compressed surfaces called the midsurface. When it models your part, Creo Simulate places elements on the midsurface only, using the thickness associated with each portion of the shell to determine the depth of the elements.

For example, if you were modeling a T-bracket, you might take the approach shown in the T-Bracket example.

Creo Simulate designates the first surface or set of surfaces you select for a pair as the top surfaces and the opposing surface or surfaces as the bottom surfaces. The top surface acts as a point of reference for the pair. Creo Simulate uses this surface as a viewpoint when determining which opposing surfaces in the model are part of the pair. In addition, Creo Simulate defines the normal direction for a compressed midsurface from the top surface to the bottom surface.

If you remove unopposed surfaces from your model, Creo Simulate compresses the model differently than if you include all opposing surfaces.