Hi Sven,

In this situation and the way Nurbs work you would model the shape of the cutout in solid and use Boolean difference to cut out that shape from the main body, eg. if you need a square pocket in a body you would model a box then subtract it from the main body.
I've shown the pieces I used to Boolean diff (Subtract) the shapes needed in the main or parent body, I have also attached the 3dm file.



Cheers