Modeling & Simulation (M&S) requirements specify the set of capabilities that a simulation needs in order to adequately serve all of its intended uses. Users rely on information from the three domains (Problem, User and Simulation) to develop a concise and consistent set of requirements for the simulation at hand. Users use this set of requirements in deciding whether to apply an existing simulation, modify an existing simulation, construct a federation, or build a completely new simulation to serve their purposes. The set of requirements consists of two complementary types of requirements: 
- Representation requirements: describe the properties and behaviors of the things that a model or simulation must represent to adequately serve the user’s purposes. These include the represented entities, their properties, and the dependencies that, when executed, produce their behavior. Representation requirements define the needed simulation capabilities in the terms of simulation fidelity. These requirements should define the resolution, accuracy, and confidence in that accuracy needed for every object property of importance for directly meeting the user’s needs.
- Implementation requirements: encompass all of those needs that are not representational in nature, such as the specifics of the interfaces and execution environments (e.g., model execution speed).
Requirements in New Simulation Development 
For each new model, simulation, or federation, a set of requirements is developed that governs how it is built (i.e., what the model, simulation, or simulation federation needs to be able to do; its capabilities). These requirements are continuously reexamined and refined as more information is acquired and trade-off analyses are performed during development and the simulation’s subsequent reuse.
Requirements in Legacy Simulation 
Reuse For reuse of a legacy simulation, the User assesses the viability of the simulation through an analysis of the existing capabilities of the simulation and its documentation. Capabilities of the existing simulation are then compared with the requirements needed to address the current problem. The results of this comparison should detail:
- Which capabilities of the existing simulation will be retained as-is
- Which capabilities need to be modified
- What capabilities need to be added to the simulation to make it fit the new intended purpose
For both new and legacy simulations, the User maintains the responsibility for articulating a set of M&S requirements that addresses the intended purpose. The User may enlist the aid of the Accreditation Agent, the V&V Agent, the M&S PM or M&S Proponent, SMEs from various applicable disciplines, other past and prospective users, and even the Developer to produce a comprehensive, correct and consistent set of M&S requirements; but, in the end, it is the User who must be satisfied that the set is accurate and complete enough to specify the simulation capabilities needed for the intended use.
Steps in M&S Requirements Development process.(see the Requirements Development section for more details)
- Requirements Identification
- Requirements Articulation
- Requirements Analysis
- Requirements Configuration Control
Important to refining requirements is managing “requirements creep” (i.e., expansion of requirements beyond those originally specified to capture the simulation’s intended uses) and to eliminate unrealistic requirements (i.e., requirements that cannot be satisfied by the available technology or resources).
See the Verification, Validation & Accreditation Recommended Practice Guide for more detailed information.
AcqLinks and References: