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When you create a spline, you can choose the "by poles" or "through points" option. The underlying mathematical foundation is the same for both spline types; but when you create/edit the "through points" spline, you do not see the pole structure. NX takes your input and manipulates the poles for you to satisfy the given constraints. I see X-form and I-form as the surface analogs to the spline's "by poles" and "through points" creation methods.
@cowski1 Thank your for sharing your observation!
In editing a "throught points" spline, only those "defining points" specified in the creation process can be modified.
In using I-Form to modify a surface, a set of iso curves can be defined freely first and then played with to modify the surface. It seems to be a little unbelievable because the final pole structure might be crazy. I guess maybe it's for some unique situations.