An arc can be exactly represented by a rational degree 2 NURBS curve with appropriate control point locations and weights. This is the form Rhino uses for arcs and circles.
Any NURBS curve can be exactly represented by a NURBS curve of any higher degree. So an arc can be exactly represented by a degree 3 curve. The degree 3 form of an arc will have more control points than the degree 2 form.
A user can transform an exact arc which is a rational degree 2 NURBS curve into an exact rational degree 3 NURBS curve using ChangeDegree with Deformable=No. The degree 3 form of an arc will have multi-knots even though curvature is continuous. It is not obvious to me when the rational degree 3 NURBS form of an arc would be preferable to the rational degree 2 NURBS curve form.
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