ExtrudeCrv documentation file (https://docs.mcneel.com/rhino/6/help/en-us/commands/extrudecrv.htm) says: "If the input is a non-planar polycurve or a planar polycurve where the extrusion direction is not normal to the curve plane, the result will be a polysurface rather than an extrusion object."
This is not true. The attached curve is planar, but ExtrudeCrv command turns it into a polysurface. (It is called open extrusion, but if you trim it, it becomes a polysurface.)
…because I have to trim it or split it… ExtrudeCrv command turns some curves into surfaces, other curves into polysurfaces. Usually rebuilding a curve changes it into a curve which makes surfaces.
Well… I can’t quite tell what is bugging you - I think you do not actually care about extrusions vs polysurfaces, it’s more about single surfaces vs poly surfaces, correct?
To the best of my knowledge, simple curves make surfaces and polycurves make polysurfaces. The only way to distinguish these two kinds of curves is exploding them. I still believe that Rhino Properties panel should provide more information about the curves, so that we do not have to explode them. I do not know what is the best way to convert polycurves into simple curves. Rebuild command usually does the trick, but I do not understand how it does it.
Hello - Rebuild creates a new curve through sampled points on on the input - the result is always with uniform knot spacing and as smooth as the chosen degree allows (as in no tangent discontinuities - they can be wobbly).
If the ExtrudeCrv command makes a polysurface, the polysurface is made of matching surfaces. Exploding the polysurface and merging the surfaces with MergeSrf command makes single surface.