When b is not zero, and relative tesselation tolerance mode is set (see USERELTESS, 6.4.17), this attribute causes that the tesselation tolerance for nurbs surfaces and curves is taken as a fraction of the diameter of a fixed box which has been defined by the user,
When b is zero, and relative tesselation tolerance mode is set, it causes that the tesselation tolerance for nurbs surfaces and curves is taken as a fraction of the diameter of the ontrol points of the surface or curve.
b must be a number node.