This affects only camera frustums. It is closely related to the "positionOnHighlightedPlane" parameter there. While you can use this "positionOnHighlightedPlane" parameter to draw the additional flexible highlighted frustum plane based on any position in the Scene, e.g. a Gameobjects position, you can instead also use this "distanceOfFrustumsHighlightedPlane" value to specify the position of the plane not as position, but as distance along the cameras forward direction. It will only have effect if the "positionOnHighlightedPlane" parameter is left at its default disabled value and if "distanceOfFrustumsHighlightedPlane" is bigger than the cameras near clip plane.