ClipFill is an option for SurfaceGraphics that specifies how clipped parts of the surface are to be drawn.
ClipFill specifies what is to be shown in places where the surface would extend beyond the bounding box.
The possible settings are:
The colors for clipped areas can be specified by GrayLevel, Hue or RGBColor directives, or SurfaceColor objects.
See The Mathematica Book: Section 2.9.11.