Constant U and V lines can be used for the graphical representation of a surface.
The iso-parameter curves are mathematically (without any approximations) on the surface. Also for instance surface/plane intersection curves are of the same type.
When such curves are further used, like for instanced used as definition curve for a new surface, it may be a good idea for performance reasons to approximate them to cubic curves with CUR_APPROX.
The iso-parameter curves may be used for quality analysis of LFT_SUR. Please refer to “Conic Lofting Surfaces”
The sample code creates U and V lines on a surface.