This setting is the percent probability that an individual, when mutated, will have one of its shapes selected at random and mutated by the replacement of a randomly selected segement of its command list with a randomly selected segment of a command list from another shape.
This setting is normalized to sum to 85% with
Command Delete Mutation
Parameter Shape Mutation
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.