The master, as the name implies, is the main governing body of our contract across the United States. Where as the supplemental is a regional contract, as in the western states for example. The main reason for this is to address such issues as state laws, geographical economic climate, cost of living…etc..Etc. The rider is further a fine-tuning of local issues pertaining to full-timers & part-timers, which has addendum’s that are further negotiated on a case-by-case basis for similar reasons as the supplemental. This is a general, very vague explanation of what these are, I realize. Perhaps somebody else will articulate it better.