Override Constraint lets you override the default behavior for constraints. The top edit box shows the strength of the tangency. The first images below shows the default tangency which has a strength of 1.0. The second image shows a strength of 0.2. Note how the surface is shallower on the blend.
The first image in the second row shows a rotation angle of 30 degrees applied to the tangency. The rotation angle is the second number box in the dialog. The second image shows Use Control Edges for the tangency control. This is available in the dropdown as shown on the right.
The first image in the third row shows Perpendicular To Surface applied to the edge constraint. The second image shows an offset distance which is controlled by the last number box in the dialog.
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