Spline on Surface
Splines sketched on surfaces include standard spline
attributes, as well as the following capabilities:
Add and drag points along the surface.
Generate a preview that is automatically smoothed
through the points.
Span multiple surfaces if the splines that created
the surfaces are tangent.
All spline points are bounded to the surface
on which they are sketched.
Use splines on surfaces with:
Part and mold design, where surface splines enable
you to create more visually accurate parting lines or transition lines.
Complex sweeps, where surface splines facilitate
creating guide curves bounded to surface geometry.
To sketch a spline on a surface:
Open a sketch on a surface model.
Click Spline
on Surface 
(Sketch toolbar) or Tools,
Sketch Entities, Spline
on Surface.
Sketch a spline on surface.
Spline need not extend from profile to profile.
Click OK
.
Related Topics
Relations
with Splines
Adding
controls to splines
Editing splines