The sample set's organ definition will define how each pipe will respond to random detuning by default (mean/maximum amount of random detuning that will be applied for the pipe, and its probability distribution). The user setting you're referring to scales the result of that, i.e. the amount by which the pipe actually gets detuned randomly will be multiplied by the user setting, so if (based on the organ definition's random detuning parameters) the randomisation model randomly chose to detune the pipe by, say, 0.3 cents, and you had set the user setting to 200% then the pipe would actually end up being detuned by 0.6 cents.
Doug S. wrote:Also, does returning to the default 100 setting undo any changes?
Yes.
Doug S. wrote:Are changes incremental or cumulative?
No -- as with all of the percentage adjustments, it just amplifies or attenuates (i.e. multiplies) the output of the model directly by whatever you currently have it set to.