ions are necessary to facilitate grounding and managing model development and simulation experiments. Understanding the specific roles of different artifacts in abstractions is important to enrich the model development process. Actions and states reveal certain unique aspects about system behavior within and among component models. However, behavior specifications are not explicit and adequately supported in state-based formalisms. In this paper, we examine the role of actions in model specification in accordance with the notion of state in the modular and hierarchical parallel DEVS formalism. The resulting approach relies on the profoundness of DEVS while having a more intuitive abstraction around it via hierarchical activity metamodeling. Together the activity and DEVS modeling help simplify and strengthen specifying complex system behaviors as composition of atomic and coupled models. We show the importance of hierarchical DEVS activity modeling approach with a coordinator model as a part of a dual-server model.