Yes, definitive: you confirm the school that corresponds to your highest-ranked application choice and where you have been admitted! All that's left is to finalize your registration in the program of your choice.
(Warning! By responding with a "yes, definitive", you give up all your other application choices)
Yes, but: indicates that you keep your application choice at the school that accepts you, but that you also retain your place on the waiting list for the programs that rank higher in your list of application choices.
No, but: means that you decline the school offered to you, while still hoping that a school ranked higher on your list will offer you a place.
(Warning! If no other school is offered to you, you automatically lose your rights to all the schools).