There are 3 types of Special Constructions in Microstation's DDD toolbox:
- B-Splines
- Constrained B-Splines
- B-Spline Cells
B-Splines are like LOCKed elements (no degreees of freedom) and can have points Constrained ON them.
Constrained B-Splines can have their ‘knots' constrained by other Constructions in order to change the shape of the B-Spline, but canNOT have points constrained ON.
B-Spline Cells CAN have points constrained ON, and can also have a directional constraint, tangental contraints and point (from its center), but canNOT have their ‘knots' constrainted.
...watch the video and it all become clear :)