When an AutoLISP application uses entnext to step through the vertices of an old-style polyline, it might encounter vertices that were not created explicitly. Auxiliary vertices are inserted automatically by the PEDIT command's Fit and Spline options. You can safely ignore them, because changes to these vertices will be discarded the next time the user applies PEDIT to fit or to spline the polyline.
The old-style polyline entity's group 70 flags indicate whether the polyline has been curve-fit (bit value 2) or spline-fit (bit value 4). If neither bit is set, all the polyline's vertices are regular user-defined vertices. However, if the curve-fit bit (2) is set, alternating vertices of the polyline have the bit value 1 set in their 70 group to indicate that they were inserted by the curve-fitting process. If you use entmod to move the vertices of such a polyline with the intent of refitting the curve by means of PEDIT, ignore these vertices.
Likewise, if the old-style polyline entity's spline-fit flag bit (bit 4) is set, an assortment of vertices will be found—some with flag bit 1 (inserted by curve fitting if system variable SPLINESEGS was negative), some with bit value 8 (inserted by spline fitting), and all others with bit value 16 (spline frame-control point). Here again, if you use entmod to move the vertices and you intend to refit the spline afterward, move only the control-point vertices.