SpeedTree artist Sonia Piasecki gives you a quick overview of the Parent Curves feature in SpeedTree v8.0.
Parent Curves are the green curves associated with many tree properties. They distribute values based on how far along the parent each child node occurs. This lets the user make parts of the tree behave or appear differently based upon where on they are growing on the tree.
Parent Curve documentation: http://docs8.speedtree.com/modeler/doku.php?id=kccurves
More info at http://www.speedtree.com