In ProTOp surface regions can only be defined as boundary surfaces of various material region combinations. Such a surface is always closed, but can be opened by subtracting imported surfaces. Finally, a complement of such a surface ban be obtained by inverting the current surface.
The figures below illustrate a part with two material regions and one imported surface; various surface options are shown.
Figure. Boundary surface of the first material region .
Figure. Boundary surface of the second material region .
Figure. Boundary surface of the whole part (both material regions) .
Figure. Boundary surface of the whole part with imported surface subtracted.
Figure. Boundary surface of the whole part with imported surface subtracted - inverted.