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 3D it’s still a curving surface.