A SubD friendly curve is a degree 3 uniform curve.

The second control points on both ends are constrained and locked. These points display with the locked object color.

The first and last three control points are always collinear to maintain zero curvature at both ends.

Moving or deleting Control Points or Edit Points will not destroy the SubD friendly characteristic.

SubD creation commands (LoftSubD, Sweep1SubD, Sweep2SubD...) create a better SubD object from SubD friendly curves.