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 consist of Bezier arcs, but they also can be defined with both control vertices and fit points, which offer different editing options.
NURBS curves have both fit points and control vertices. The fit points are directly on the curve, and the control vertices are outside the line. Use fit points to make small, localized changes to a curve; use control vertices to make changes will affect the shape of the curve as a whole.
NURBS surfaces and curves can have a clamp, closed, or open form. The form affects how the object can be modified.