The "driver" represents the UE side of the NetworkPrediction System. Typically the driver will be an Unreal Actor or ActorComponent. This driver class is defined in the ModelDef (your version of FNetworkPredictionModelDef). For example, AActor or AMyPawn class. While the Simulation class is agnostic to all of this, the driver is responsible for specifying exactly how things should work, generating input and handling output from the system.