Assemblies of people are gatherings where persons are so densely packed that an individual cannot move away from the crowd — concerts, rallies, sports events, beaches in season. The decisive criterion is not the number of people but the density: can a person freely step aside.
Flying over assemblies of people is prohibited in the entire open category — regardless of the drone's class or mass. Even the lightest C0 drone, which A1 allows to overfly individual people, may not overfly a crowd. That requires the specific or certified category.