RebuildCrvNonUniform documentation is here: https://docs.mcneel.com/rhino/7/help/en-us/commands/rebuild.htm#RebuildCrvNonUniform
Is says: The RebuildCrvNonUniform command interactively modifies selected curves by non-uniformly re-spacing the control points.
In reality the RebuildCrvNonUniform interactively modifies end points of the curve only. All other control points are somehow influenced by single sliding point located in the middle of the curve. This single sliding point is almost useless. What we need is the ability to control density of the control points along the curve. dense spacing = more detail; sparse spacing = smooth shape