Simple surface from simple planar curve isn’t planar

That depends on the situation specific and associated definition of “brep”.

The parasolid format has it’s own vocabulary. It’s similar to word usage in different every day language. If an American goes into a building labeled “hotel de ville” in France they will be surprised to find it is the town hall, not a place to sleep.

There are NURBS curves with 1 parameter, NURBS surfaces with 2 parameters, and NURBS volumes with 3 parameters. Rhino uses NURBS curves and NURBS surfaces but does not use NURBS volumes.

In Rhino land a surface without a modifier usually means a NURBS surface or a NURBS polysurface. There may be a few inconsistencies which have snuck in.

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