The SD accepts earlier lenses, backs, etc.
Newer KL lenses fit Pro or Pro S if you remove the adapter ring from the back of the lens, (except the 75 & 500 as far as I know - I don't own these myself).
Mamiya say older lenses should have adapter ring fitted to avoid possible light leaks with SD body - I don't bother and have had no problems.
To shoot 6x8 with Pro S a new, larger rear plate needs to be fitted to the body and a 6x8 rotating adapter used. The SD (I think) has these as standard - mine have.
I really like the 6x8 electric backs, make the RB much easier to use.
I do own a Pro as well as S's & SD's but later rotating adapters don't fit because of the interlocks. So my Polaroid back lives on the Pro. No adapter needed. The Pro doesn't have focus lock or vertical/horizontal frame indicator in the finder either.
The SD body can use SD or earlier extension tube but the Pro & S can't use the SD extension tube - it's made to the size of lens + adapter and is REALLY ANNOYING when I pick it up by mistake to use with an S.
Like everything else they do go wrong sometimes but they're easy to repair.