# Systematic Literature Review of System Models for Technical System Development
## 4 Discussion
### Usage of the Term 'System Model'  
In regards to the use of system models, 51% of sources indicate a primary use of system models in the context of their publication as analytics, as opposed to 49% of publications that indicate system synthesis as the main driver behind the application of system modeling. According to our observations, this unclear picture is largely driven by the nature of systems development in engineering. Due to the recursive and iterative nature of system development, simulations as an aspect of system analytics generate knowledge about a current or future system, yet might ultimately be driven by system synthesis. This circular dependency between analytics and synthesis also means that the results obtained are usually applied to further develop and optimize a system until a desired system maturity and layout is achieved through multiple iterations. This usage is not bound to a single domain but is widely spread as could be seen in Figure 6.  
One question we tried to answer when we set out to review literature pertaining the definition and use of system models was whether there is consensus if there can be more than one'system model' per system. It turned out that even within individual publications determining whether a single or multiple system models are being developed or applied is very difficult, due to the generally iterative and recursive nature of system development. None of the selected publications put much emphasis on this question either. The first issue here may be the vague definition of what constitutes a single model versus a group of highly interconnected models. For example, there is not even consensus on a technical level whether multiple diagrams in a graphical modeling notation constitute one model or multiple ones. This, again, may be attributed to the fact that system modeling is often applied from a need-driven perspective and ultimately it is probably not important as long as project/product boundaries are predetermined and the selected modeling approach supports existing or prospective use cases. This is further supported by the fact that none of the analyzed publications explicitly defines clear boundaries between pre-domain systems modeling and domain-specific modeling and development approaches. None of the samples attempts to even implicitly define a generalized definition of that pre-domain/domain boundary, which suggests that this boundary may be driven by existing processes and organizational structure and therefore be highly dependent on a specific use case. None of the reviewed publications contains negative views on system models, despite some samples mentioning new difficulties which arise with new methods and tools, such as requirements regarding IT-infrastructure, potentially new organizational structures as well as extended skill sets of developers. As conclusion, we found no consensus across the literature body, if there can be more than one'system model' per system. Looking at the different definitions used in the publications (see Figure 8), there seems to be no evidence that there must not be more than one system model per system. In conclusion, this means that multiple system models per system should not be considered infeasible per se and might very well be useful depending on individual use cases.