This setting is the percent probability that an individual, when it undergoes a Command Change Mutation, will have the the randomly selected command changed completely. It may become a different type of command and its parameters will be fully randomized..
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 Command Parameters to a low value.