Makes the end of a curve tangent or curvature continuous with another curve or surface.
Edit Curves > Project Tangent > 
Tangent modifies the curve by projecting its tangent vector where it intersects the surface onto the tangent plane of the surface. This means that only necessary modifications are made to the start or end of the curve where it intersects the surface.
Curvature makes the curve tangent and the curvature continuous with the surface in the direction of the tangent vector. An extra manipulator is displayed on the curve to let you adjust the Curvature Scale value. (The Curvature Scale slider is also displayed in the options window when this option is selected).
The Tangent Align Direction options provide a convenient way to either reverse the direction of the curve’s tangent vector or align the tangent vector with the U or V parameter directions of the intersecting surface or two curves.
U and V pick the tangent you want to use for the adjustment. U is the U direction of the surface, or it’s the second selected curve. V is the V direction of the surface, or it’s the third selected curve.
Normal is the normal vector of the tangent plane. Select the Normal option to make a curve normal to or perpendicular to a surface or two curves. When selected, the curve is no longer tangent to the surface since it is perpendicular to the surface.
When you select Normal, it becomes the mode you are working in for the current curve modification. To return to the general project tangent operation, select either the U or V tangent align direction.
The Tangent Rotation slider displays the current tangent rotation angle. Adjusting the rotation rotates the tangent vector on the tangent plane defined by the surface intersection.
Tangent Rotation is not available when the Tangent Align Direction is Normal. If you change the Tangent Rotation value, the curve is no longer tangent to the surface or two other curves.
Project tangent works by selecting a curve to modify then by selecting a surface or two other curves that intersect with either of its end points. The curve is modified by projecting its tangent vector where it intersects the surface onto the tangent plane of the surface.
If Curvature is selected as the construction type, the curve is made tangent and the curvature is made continuous with the surface or curves in the direction in which the tangent vector is going.
Adjusting the Curvature Scale value modifies the curve in such a way that the tangent or curvature doesn’t change at the curve or surface intersection point. For example, if you modify a curve at its end point, the curvature slider moves the third CV from the end of the curve along a line that joins the end two CVs of the curve (for instance, along the tangent vector line). Such a modification doesn’t change the curvature at the end of the curve, it just adjusts the tangent vector.
