When you draw a rectangle there are actually 8 points and 4 coincident constraints. When you extrude the rectangle it creates 8 line entities perpendicular to the sketch plane from those 8 points (it should be 4). The NURBS shell is created by sweeping the line segments into planes/faces but the curves connecting those faces are created from the points on the faces themselves - one curve per face. Somehow curves get sorted in terms of direction so we know to always use the first (or maybe it's last) point of the sketch curve to create the curve/trim joining faces.

If you subsequently constrain to one of those lines, it'll be random which of the overlapping entities you constrain to. I did this with 5 random line segments (each attached after rotating the model). When you delete one of the 2D rectangle lines and replace it, the deletion will delete the constraints attached to the corresponding extruded lines. In my case 3 were on one line and 2 on the other.

If we add revolve arcs in the same way as in #1631 every point becomes an arc. we will be making duplicate arcs just like the case with lines.

But who cares!?!??! Well my plan for the future is to tag those NURBS shell edges with the corresponding entity handle. That way we can introduce edge modifier information (fillet, chamfer radius) to the entities and then apply the modification to the NURBS shell edges. But this will require getting rid of the duplicate entity to avoid confusion. Regardless of how it works, we are going to want a one-to-one correspondence between SShell curves and entities, though some edges still may not have entities.