This option is for modifying the vertical alignment of a road centreline, by adding or modifying a vertical parabolic curve (filleting a vertical IP).

On the road tools toolbar in a longsection window, click the add curve button.

Select a vertical IP (they highlight as you move the mouse cursor around the longsection view).
Then move the mouse left/right to adjust the vertical curve length.

You will see the parabolic curve adjust as you move the mouse.
Click the mouse to place the vertical curve.

Curve Length / K Value / R Value

You can enter an exact curve length (or K Value or R Value) by right clicking the mouse to get the menu, and selecting Enter Length / Curve Parameter

The length is the horizontal distance from the start to end of the parabola.

The K Value is a measure of the rate of change of curvature with distance (mathematically defined as the parabola length divided by the change in the gradient from the start to end point).

A parabola does not have a true radius. The R Value is the K Value times 100.

You can change the road centreline alignment at any time during the design process. Site3D will work out all the knock-on effects and update your site accordingly.