The SideFill fields are only valid for visible curves. With respect to the HiddenLineDrawing, the region to the left (or right respectively) of this curve is described by CurveSideFills[0] or CurveSideFills[1], respectively. If exactly one of these regions is empty this is a scene silhouette. If this region contains a surface it is either a surface which in 3-D is adjacent to this edge or it an surface that is further away from the camera, we call this a shadow surface. unknown is used for unset values and for projecting curves