The projection tool has three modes. In the first mode you can use a planar object to split a NURBS curve. In the picture I have used a rectangle to split a curving NURBS surface. In the final image you can see that I have separated the original NURBS surface from the split one. Notice how the rectangle has split the NURBS surface so that the plan view is still a rectangle but in 3-D it's still a curving surface.
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