High expectations to leverage Modeling and Simulation (M&S) capabilities to support operational testing, systems analysis and user/operator training is motivated by a number of factors. As the complexities of new operations, such as multi-domain operations and operations in highly contested area operations, can no longer be tested and trained in live environments, leveraging M&S capabilities efficiently is paramount. In this paper, we use the differentiation of combat simulation requirements in terms of functional (e.g., model fidelity) and non-functional (e.g., real-time, distributed) characteristics to define a trade space to highlight challenges in relation to simulation purpose. 5^th generation weapon systems require lots of data with high fidelity, depending on the purpose of the simulation. As functional and non-functional characteristics are interdependent, the design of supporting simulation systems must be well aligned with the purpose when developed, and is constraining possible purposes to be supported once it is developed.