class AcDbSpline : public AcDbCurve;
This class implements the AutoCAD SPLINE entity.
Objects of the AcDbSpline class use an embedded gelib object to maintain the actual spline information. The spline itself may either be a simple curve fit (within a specified tolerance which may be 0) through a set of "fit points," or it may be a NURBS spline (that is, a set of control points, knots, and weights used to define the spline path). Internally, a curve-fit spline still has NURBS data; however the reverse is not true.
The following books are a good place to start to get a basic understanding of spline curves:
The AcDbSpline class provides functions to query and set a curve-fit spline and a set of functions to query and set a NURBS spline. It is not recommended that the curve fit functions be called on a NURBS spline or that the NURBS functions be called on a curve fit spline. Invalid data, or worse, may result. The AcDbSpline::hasFitData() method can be used to determine which type of spline is in hand.
Creating a NURBS spline with a sequential series of identical control points results in an unusable spline due to continuity problems.
SPLINE in the AutoCAD Customization Guide and AutoCAD Command Reference