S=selfed; i.e. chemically treated to mutate, to cause sexual organs of the opposite sex to appear and results from those are used. Typically it's a female that is 'selfed', that is to say chemically treated so as to cause viable male organs and pollen to be generated. By that pollen the original female is bred. The numbers denote generations.
I am likely not using the proper terminology, the above explanation is my take on what I've read on the subject.