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As you originally have created the spline in a specific view with all the defining points on the same plane the resulting spline curve is planar. The intuitive editing functionality is that the points should remain in the same plane as the creation (this is in most cases the desired behaviour and really anoying when not maintained). However if you have defined the defining points for your spline in true 3D coordinates the resulting spline is a true 3D curve and have no plane to relate to why you have full freedom from start to edit. Its an easy switch to change the behaviour of the spline definition points. I agree that maybe the constrained plane option should maybe should be turned on upon edit..... mabe an ER?
Hi @Sandman Thank you for you explanation!
Here in this situation, NX seems to be too smart Maybe most people like this and don't want Constrained to Plane to be turned on. Let me settle on the current behavior.