Thicken

Normally, you model objects with only the outer surface visible to the camera. However, if the surface is broken through, the object should be output to a 3D printer, or of certain render effects require a suitable volume (e.g., Subsurface Scattering), this "paper-thin" surface must be given a wall thickness. This is done by vertically moving a duplicated surface:

The wall thickness can be affected via Vertex maps.

Since Selection tags are assigned internally for the original surface, caps and the border, these can each be assigned a separate material.

Note also the Thicken tool that interactively does something similar.