For print.objsurf, print.summary.objsurf,
plot.objsurf and image.objsurf the value is NULL.
For contour.objsurf and persp.objsurf
the value is described in the help for
contour.default and persp.default
respectively.
For summary.objsurf the result is a list, of class
summary.objsurf, containing summary information. This list is
printed in sensible format by print.summary.objsurf.