Autodesk CIVIL3D - CIVILDESIGNCOMP 2007 SLM CD Tutorials Manual page 72

Contour Data
Contours are graphical illustrations of surface elevation changes. You can
create a surface from contours drawn as simple 3D polylines, which have x,
y, and z coordinate data.
Boundaries
Boundaries are closed polylines that affect the visibility of the triangles either
inside or outside the polylines. An outer boundary defines the extents of the
surface. All triangles inside the boundary are visible, and all triangles that are
outside the boundary are invisible.
Areas hidden by boundaries are not included in calculations, such as total
area and volume.
Surface boundaries are defined by selecting existing polygons in the drawing.
The surface definition displays the numerical ID and a list of vertices for each
boundary.
60 | Chapter 5 Surfaces Tutorials