# Systematic Literature Review of System Models for Technical System Development
## 4 Discussion
### Drivers and Indicators for the Usage of System Models  
The decomposition of the statements on the reasons for applying system models into indicators and drivers supports a cause-and-effect analysis between drivers of system model usage. This approach connects the question why system models are being considered (Drivers) with the question, which measures authors aim to invoke on a technical level in order to achieve what they set out to accomplish (Indicators). Since the body of literature is of the size n = 104, most publications mention only one driver (111 mentions of drivers and 143 mentions of indicators) and for readability, the numbers shown in the Figure 11 represent the share of the drivers and indicators mentioned in the publications over their respective sum in absolute numbers. The different indicators are comprised of clustered aspects of systems development in engineering, which are supposed to be optimized according to the publications contained in our body of literature. The drivers are comprised of system properties on one hand and perceived challenges across a systems development life cycle on the other hand. Potentially perceived challenges might trace back to thesystem or product properties, but there was no clear evidence for this in the analyzed set of publications. While beyond the scope of this review, the obvious fact that system development activities seek to produce a system that exhibits a set of desired properties, suggests that the drivers would ultimately all trace to the system or product properties (the \"best\" possible system). The flow within the figure highlights relationships between drivers and indicators. If, for example, a publication describes the impact of improved traceability and attributes this to the driver Collaboration, this is recorded as a relation and is displayed as a sankey flow in the figure. The width of each sankey flow connector correlates to the number of samples mentioning this driver-indicator relationship. This enables visual identification of the correlations between drivers and indicators, and indicates the frequency of occurrence in the literature body. The drivers were aggregated to form groups from the sum of all identified drivers contained in the body of literature, which often used different verbiage but was alluding to the identical driver:  
* _System Complexity:_ By far the most important driver resulted from the focus of many publications on improving the development and operation of large and highly interconnected mechatronic or cyber-physical systems. * _Development Process:_ A large number of publications included in the body of literature indicated the development process itself as the main driver for the application of system models in order to maintain consistency across processes and methods that are themselves complex and can not be handled well without the extensive use of modeling. * _System Quality:_ This is perhaps the most basic of all mentioned drivers and refers to the quality properties of a developed system as opposed to the performance of its development lifecycle activities. * _System Design:_ This driver pertains to the functional properties of a system and is mentioned by publications that describe the development of new features and design solutions, which emerged using system modeling. * _System Safety:_ The publications that explicitly describe safety as one of the drivers behind the use of system models employ systems modeling as a means to derive safety engineering-related artifacts automatically (e.g., fault trees). * _System Validation:_ This driver relates explicitly to system validation activities. * _System Modularization:_ Publications that mention this driver view system modeling as a tool to improve system modularization in terms of clear and standardized system boundaries to support compatibility with other systems and sub-systems. * _System Security:_ This driver relates systems modeling to the development of secure systems.