Autodesk AUTOCAD 2006 Command Reference Manual page 870

EXTEND changes the frame by adding a new vertex where the initial or
final line of the frame intersects the boundary geometry.
BREAK and TRIM generate a polyline with only the fit spline, which is
consistent with fit curves, where the curve fitting is permanent.
EXPLODE deletes the frame and generates lines and arcs to approximate
the spline-fit polyline.
OFFSET generates a polyline with only the fit spline, which is consistent
with its behavior with fit curves.
DIVIDE, MEASURE, and the Object option of AREA and HATCH see only
the fit spline, not the frame.
STRETCH refits the spline to the stretched frame after a spline is stretched.
The Join option of PEDIT decurves the spline and discards the spline
information of the original and any added polylines. Once the Join operation
is complete, you can fit a new spline to the resulting polyline.
The Edit Vertex options of PEDIT have the following effect:
The Next and Previous options move the X marker only to points on the
frame of the spline, whether visible or not.
The Break option discards the spline.
The Insert, Move, Straighten, and Width options automatically refit the
spline.
The Tangent option has no effect on splines.
Object snap uses only the spline-fit curve itself, not the frame. If you want to
snap to the frame control points, use PEDIT to recall the polyline frame first.
The SPLINETYPE system variable controls the type of spline curve
approximated. Setting SPLINETYPE to 5 approximates a quadratic B-spline.
Setting SPLINETYPE to 6 approximates a cubic B-spline.
quadratic B-spline
846 | Chapter 17
cubic B-spline