The spline tool is optimized to work with NURBS modeling
NURBS surfaces can be created from a number of 2D objects, including edge subobjects, polylines, and arcs. But the spline tool is the only object that has options that are compatible with creating NURBS surface. Not only do splines consists of Bezier arcs, but they also can be defined with both control vertices and fit points. The fit points and control vertices offer different editing options such as knot parameterization and degree options. See
Moving Fit Points vs. Moving Control Vertices
NURBS curves have both fit points and control vertices. The fit points lie on the line, and the control vertices lay outside the line. Use fit points to make a change to one small part of a curve; use control vertices to make changes that will affect the shape of the curve as a whole.
Clamp Surfaces and Curves with Open and Closed Geometry
NURBS surfaces and curves can have a clamp, closed, or open form. The form affects how the object deforms.
