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• Flat Curves - usually recommended for blades of grass or other geometry where the curve geometry is relatively more "ribbon-like" than tube-like in appearance. User-normals are specified for flat curves that will twist around so that their orientation always follows the user-specified normals (for both camera and other ray types). No-normal curves (always camera facing) are not supported. Flat curves are no appropriate for foliage or other "flat" surfaces where the  geometry is roughly rectangular and the desired silhouette is obtained with a cut-out texture. For these it is much better end more efficient to use polygons instead.
• Round Curves - typically recommended for hair and fur since this usually represents the underlying hair geometry with greater fidelity (although not in all cases), and as a direct consequence round curves do tend to produce a noticeably more accurate reproduction of occlusion and shadowing for fur and hair compared to flat curves. User normals must not be present when using round curves; if user-specified normals are provided, currently the user-normals they will take precedence over the round curve attribute default and as a result the curve will instead be rendered as a twisting ribbon-like curve that follows the user-specified normals (e.g., a flat curve). Currently, round curves tend to exhibit better time and memory performance compared to flat curves. It is highly dependent on the BxDF and shading model, but in some cases, the tube-like varying normal produced when using round curves can yield qualitatively more interesting shading results.

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