One of the most obvious differences between a class and mass action are the number of plaintiffs named in the lawsuit. Class actions have only one or a few named plaintiffs, who act as representatives of the entire class, because, as Trial Rule 23 says, the class is so large joinder of all the members would be impracticable. That means technically there are only a few named parties to the lawsuit, and those class members not listed as class representatives have very limited input and responsibilities with regard to the lawsuit.
Those absent class members normally don’t have to answer discovery, go to depositions or court hearings, and don’t have to prove their claims individually (at least not their liability claims). Instead, although there are some chances along the way for them to opt out or contest any settlement, for example, even class counsel only has to communicate with class members during certain limited times during the lawsuit. The other side of the coin of this representation, however, is that if the class is certified the results of the lawsuit are res judicata for all class members — whether named or not. That is why to get certified as a class the court must determine that the class is adequately represented by both the class representatives and the attorneys for the class.
On the other hand, mass actions typically have many allegedly injured parties too, but instead of choosing one or two people to represent everyone, each person is individually listed as a plaintiff in the lawsuit. Since the Class Action Fairness Act (CAFA) was signed into law in 2005, whenever such a mass action lists over 100 plaintiffs though, it is typically deemed to be a class action removable to federal court by defendants. 28 U.S.C. 1332(d)(11). Further, when many lawsuits, even across districts, deal with the same issues they are often brought together with federal multi district litigation, governed by the procedural rules found at 28 U.S.C. 1407 and 2112.
Unlike with class actions, each plaintiff in a mass action has responsibilities for doing all the activities that any regular plaintiff must do, like discovery, etc. Further, although there are some procedural mechanisms that can be put in place to try to prove some elements of the case across the board for all claimants, at the end of the day each person’s case ultimately is decided individually.