“You can be owner of 10 accounts and act differently on each as owner of that account - with the associated shares held there in”
Yes. But the choice has an important and mutually exclusive effect. If you opt out you *cannot* recover from the class itself—whether it settles or wins. This rule is an *absolute bar* in America (note: shares held outside the US may be subject to different laws).
So for example, say you opt in to the Certified Class on accounts 1-9, and opt out on #10. Account # 10 can *not* recover on any class settlement; #10 can only sue individually.