NX generates knots automatically in a multi-segment spline. For example a 2-segment spline has a knot between the start and the end, and generally % parameter of the knot is not 50%.
Is there any easy principle for judging % parameter of knots in a spline?
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We have changed the algorithm a few times over the years, and, to be honest, I haven't looked at it for a long time, so I don't know what the code does today. But, whatever it does, you can't rely on it, because we'd change it if we discovered a better method.
The most common approach is to base the knot values on chord lengths. So, the parameter increment between points is proportional to the chordal (straight line) distance between them. Some of the available techniques are explained here.
In NX/Open and SNAP, you can actually input the knot values (and node values), which allows you to produce a wide variety of different shapes. But these controls wouldn't make much sense to most users, so they'e not exposed in the interactive system. To understand what you can do, you could start by reading pages 53-55 of this document.