This setting is the percent probability that when a shape undergoes a Parameter Shape Mutation, it will mutate the Z-Widening Parameter. Z-Widening determines to what ratio of the base size the top curve should flare to when the shape is extruded.
Widening is normalized to sum to 100% with
when you leave this dialog page, so it may not retain the same numeric value when you return, but its relative value will be equivalent. That means that to set this value as high as you can, you must set the others listed above to low values.