Hi vinced, here's a bit more description on "trim curves" and "underlying surface".

Here I've drawn a plane, using the Draw solid > Plane command:



If you then turn on control points for that plane, you'll see it has 4 points, one point at each corner:



Now I draw in some curves in the middle of that plane:



Now if I trim that plane with those curves, I get something that looks like this:



However, if you turn on the surface control points, you will see that the surface itself still just has 4 points in it at the corners, same as it did at the start. The hole is defined by a set of "trim curves" that are on the surface, but as you can see they can be going in some totally different direction than the surface's own control points.

The thing that controls the actual shape of the surface are the surface control points, the trim edges that you see there are just marking portions of the plane as being a hole. So there isn't a connection from being able to yank on one of those edges and have the surface shape itself be manipulated, you would only be yanking around the thing that marks which parts of the surface are active or inactive.

This is a way different structure than how polygon modeling works.

- Michael