This setting is the percent probability that when a shape undergoes a Parameter Shape Mutation, it will mutate the ShapeMode parameter. ShapeMode determines how this shape should behave when it intersects with another shape? It may intersect, union, subtract from, be subtracted from, etc. There are 45 options which are not clearly identified in this documentation.
ShapeMode 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.