What is the difference between estoppel and cause of action?

That said, there is a subtle difference in focus between the two branches of res judicata. Cause of action estoppel is focussed primarily on fairness to litigants. The idea behind it is that a party should not be “twice vexed” with litigation, and should be entitled to deal with the entirety of the opposite party’s case within a single piece of litigation. Issue estoppel, on the other hand, as discussed in Toronto v. C.U.P.E., is primarily concerned with the integrity of the judicial system – the efficiency of the trial process and the authority and credibility of judicial findings.