Hi mir4ea - once you have a closed surface the control points from either side of the closing seam are displayed as one common control point and when you move it the real internal control points from both sides move together. That's so that if you have a closed surface it will stay closed as you edit the control points, otherwise if it didn't do that it would easily pull apart and become not closed anymore if you only moved one of those points.

The same thing applies to closed curves as well.

There is also a different kind of closed curve or surface called "periodic" closed - that's when the curve or surface is smooth at the closure point instead of coming to a sharp point at that location. If you draw a circle or a sphere and turn on control points for those and move those around you'll see that kind of behavior.

- Michael