Changes how curvature is calculated at the picked blend curve constraint.
Two tools are available on the Constraint Curvature Type pull-out: Blend constraint geometric curvature, and Blend constraint parametric curvature.
Parametric curvature
Align the curvature to an isoparametric direction of the surface.
This type of curvature is especially useful to achieve curvature continuity with the edge of a surface.

Geometric curvature
Construct a plane from the surface normal and the constraint direction (or the normal and the tangent of the curve at the attach point for location constraints).
Create a section curve by intersecting this plane with the surface.
Align the curvature to this section curve.
