1. During the course of creating curves & features , you may get a message "curve does not lie in x-y plane"

2. Go to the curve wizard>curve from curve>project to UCS ( user coordinate system)  Select the plane (XY, XZ, or YZ ) that you want to project (or "flatten" ) the curve to.

3. Since the program carries 6 decimal places , it's possible to have a piece of geometry laying at z=0.000001 and the z position displayed will read z=0.0000.  This can lead to a message such as "doesn't lie in XY plane. 

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