Although Alias can handle NURBS, geometry created within Alias using the default construction settings (Rational flags off) are all based on non rational B-splines. Some functions like "Align" does not behave properly when mixed with ration and no-rational geometry.
Is it necessary to use rational geometry? Analytic surfaces and curves, such as circles and surfaces of revolution can be easily defined by rational geometry. If one need such precise geometry one can use rational geometry. But better to use them sparingly and at the last stage of the project .. like small fillets etc.
The non rational geometry can be again of two types .. chord length based and parameter based. Again it is not advisable to mix them. With explicit geometry control for most functions, it is not very critical but generally one should stick to either one of them. Blend curves by default creates chord length based curves.
In my experience if using multi-span cubic geometry, chord length based geometry gives better internal continuity/smoothness.
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