Creating a spline by first creating a polyline and the using pedit S will create a degree 3 B-Spline. Note that the property box will show the spline as being a 2D Polyline. This type of spline was AutoCAD's first implementation of splines in the 1980s. The number of chords that define the spline is a function of the SPLINESEGS command. The default value is 8. After creating a spline from a polyline change splinesegs to 2, use pedit to Decurve then Spline the polyline again and you will see a much more coarse version of the spline. Using a larger value for splinesegs will display a smoother curve.
If you edit the polyline spline with the splinedit command the spline will be converted to a NURBS (non-uniform Rational B-Spline). The spline will appear more smooth and you will gain many options for editing the spline.
Although a spline my have just a few "control points" (a.k.a. Control Vertices) the offfset command will usually generate a spline with many more CVs that its source spline. However, if the pedit-spline was a "fit" spline then the curve is not really a spline but a series of arcs end-to-end and an offset curve will be a pline composed of arcs.
I recommend not using polylines to construct a spline. Use Spline Method Fit to create your first draft of a curved then use splinedit to modify at least one CV (or add a cv). After this you can easily move CVs to adjust the shape of the spline by just selecting the spline and picking a CV to move.
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