What? Yes! S-bonus is paid to bonus account which actually is just like regular bank account. Technically it is one. This was due to some law changes some time ago which forced the change.
Bonus goes to this account from which you can get it out in actual cash. Or you can use S-card to pay purchases in S-ryhmä when it is charged from your "bonus" account.
Plussa system in turn is not based on account with money but they send you the coupons which give discount at cashier. Essentially coupon pay X-euros from your purchase. IIRC it cannot be traded into cash but only used to pay purchases in K-ryhmä.
You can get Visa added to your S-card so you can pay with it elsewhere as well with credit. Not really sure if it works as debit card outside S-chain.
Plussa in turn can be added to your OP-Pohjola back card. So it can result in situation where your OP (osuuspankki, bank in Finland for those unaware) card can be debit card, credit card AND Plussa card. All in one.
Which can make it amusing if you go to S-chain store, give them your S-card and then pay with OP-card which has bit Plussa-logo in it.

Ok, it's not THAT big. But obvious still.
And these days you often enough just stick the card into reader so cashier hardly sees it. But still.