# Systematic Literature Review of System Models for Technical System Development
## 3 Results
### Description of the Literature Body
This may be explained by the fact that holistic system modeling is often either not applied to established system development processes or simply just not recognized as such, driven by the fact that organizations often develop system modeling capabilities over time and through a need-based bottom-up approach. 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 a consensus if there can be more than one system model per system. Most of the publications (72%) refer to system models as a conglomerate of multiple models. In some cases (28%) the term'system model' is used for a specific type of model that can be used without further dependencies or related models. The definition and purpose of the system model have been extracted from each publication as well. Due to readability, the table has been added to the Appendix A (Table A6). For each reference, the extracted key points for the purpose of the system model in the publication as well as the definition in sense of what is inside the model and how is it is created are listed there. The purpose has been clustered as synthesis and analytics in Table 2.  
Both use cases, analysis and synthesis, make up roughly the same percentage with analytical use cases having a slightly larger share (51%). To get a better insight, these aspects will be further investigated in the discussion part. Regarding the definition of a'system model' the distribution taken from Table A6 are listed in Figure 8.  
The most widely used definition of a'system model' are graphical language models defined with SysML or Object-Process Methodology (OPM) (44%). 24% of the literature body call the combination of different domain-specific models a system model.Explicit domain models used for simulation like Matlab models are used in 14% of the literature body when speaking of'system models'. Eventually, pure mathematical models as differ  
Figure 7: Distribution of maturity categories in literature body. ential equation (DEQ) systems and data models are meant in 10% and 4% of the literature body, respectively. While most publications regarding graphical language models had references to MBSE, the publications presenting mathematical models and domain models did often not mention MBSE at all. Figure 9 displays the dominant model formats for these definitions. Regarding the primary model format a publication utilizes there is a wide variety of custom or commercial tools and formats and only very few formats are used in more than three samples. SysML and Matlab/Simulink or Modelica are the dominant ones across all systems model definitions. Analyzing the primary model formats used in publications in correlation to their understanding of system models shows that multiple samples depict graphical modeling as the main aspect of systems modeling, but utilize Matlab/Simulink or Modelica as the primary model format. This is mainly caused by the fact that a large part of the publications describing graphical modeling as the core of a systems model, connect various behavior models through graphical diagrams. As MBSE is largely driven by Software Engineering, the distribution of software systems as target system, compared to interdisciplinary systems has been investigated and is shown in Figure 10.  
Figure 8: Number of definitions used in literature body (multiple assignments possible). Table 2.0 Number of model types per definition in literature body (multiple assignments possible). The distinction between publications that focus on the information system or software-driven aspects of their system of interest on one hand or the entire system across all domains equally on the other hand, interestingly does not show huge discrepancies in the respective understanding of system models. Data Models as the focus of systems modeling are significantly more common in software-centric publications compared to more holistic ones.