This setting is the percent probability that an individual, when undergoing a Command Change Mutation, will have one of its shapes selected at random and mutated by the changing of a command in its command list.
Either the command will have its parameters changed or it will be randomized further and may become another type of command. The probabilities for each is settable in the controls below this one.
This setting 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 Fully Randomize to a low value.