When b is not zero, this attribute causes that the tesselation tolerance for nurbs surfaces and curves is treated as a relative number; it then either gives a fraction of the diameter of the bounding box of the control points of the surface or curve, or a fraction of the diameter of a fixed box which has been defined by the user (see USEBOXRELTESS, 6.4.16).
When b is zero, the tesselation tolerance is taken as an absolute number (a length in the world coordinate system).
b must be a number node.