  This paper presents our findings for using activity modeling for simulation
(validation), model checking (verification), and execution purposes. Each is
needed to tackle system complexity and further research into behavioral
modeling. We argue different models implicate different understandings and
expectations. We emphasize some distinctions with demonstrations using the
Discrete Event System Specification with an exemplary model. In particular, the
continuous-time base in models helps observe some inherent limitations and
strengths in acquiring each capability. The temporal characterization of input,
output, and state, or the lack thereof, is at the core of developing behavioral
specifications. We use DEVS to arrive at the capability of validating
simulations for activity models. We use Constrained-DEVS for the verification
of activity models. We show how some executions can be derived, whether from
the specification itself or with considerations for simulation and model
checking.
