What is SubD friendly?

A NURBS curve is SubD friendly if

• it is cubic (degree 3)
• it is uniform
• it is non-rational
• it is periodic or has zero 2nd derivative at the start and end.

A NURBS surface is SubD friendly if

• it is bicubic (U and V degrees are 3)
• it is uniform (U and V knots are uniform)
• it is non-rational
• has no poles (singular points)
• in the "u" parameter direction it is periodic or has zero 2nd partial derivative with respect to u derivative along its "East" and "West" edges.
• in the "v" parameter direction it is periodic or has zero 2nd partial derivative with respect to v along its "north" and "south" edges.

The difference SubD friendly makes

When the ToSubD command converts a SubD friendly surface into a SubD object, the SubD edges will match to the surface isocurves.

When the input surface is not SubD friendly, extra rows of SubD faces will be added to get a closer match.