## 2 Research Background
### Understanding of the Digital Twin  
#### 2.2.1 Definition of the Concept  
The origin of the concept of DTs goes back to [PERSON] in 2002. The DT concept comprises a real and a virtual system, as well as the connections for the flow of data and information from the virtual space to the real and vice versa. [PERSON] describes the existence of a real or potentially real physical system, which is the basis for the newly created virtual system, as a central prerequisite for the concept [9]. To date, there is no uniform definition of the DT concept [28] and many further developments and different understandings of DTs exist [18; 29]. Due to the multitude of definitions and the resulting inconsistent conceptual use, it is difficult to describe the main components of a DT [30]. These ambiguities have led to some difficulty in clearly delineating conceptual similarities such as the _digital shadow_ or _digital model_[16; 27]. This is due to the fact that DT has evolved technologically and there are different areas of application, which have also expanded [29; 31]. Furthermore, the extent to which the DT is integrated into other systems is decisive for the definition [27]. The definition of DTs is, therefore, application-related and specific. In general, based on the literature and industry, it can be concluded that the overarching goal of a DT is the provision of services [30; 32]. In comprehensive studies on the current state of research on DTs [17; 18; 29], provide an overview of commonly existing definitions [33]. In summary, the digital representation and information exchange between physical and virtual space are important artefacts. Furthermore, the use of five dimensions in the conceptual description and implementation of DTs in the context of implementing a DT in a production facility are recommended. According to this, a DT consists of a physical and virtual space, data, services, and the connections between the individual dimensions. This conceptual description is confirmed by [29; 34]. In addition, with reference to the functionality of a DT, the DTs include the reflection of real-time processes--an interaction and convergence as well as self-development. Furthermore, the majority of published studies and research focus on the implementation of industrial applications in production [18; 27; 35]. They summarize the primary fields of application such as the design, production, forecasting and maintenance of a product, and therefore, exist along the life cycle of a product or asset [29]. In addition, to summarize, DTs have promising advantages along the product life cycle in industry, and furthermore, they have great potential for the realization of Industry 4.0. In practical use, DTs are still at the beginning of their widespread implementation. Among industrial and IT companies as well as IT market research institutes and consultancies, different definitions exist, as well. For example, Gartner defines processes, organizations, or people, besides the initial the concept of a DT, as relevant components, and the focus of the virtual mirror image. The definition of a DT is steered in the direction of an ecosystem, as the data of different DTs are linked and aggregated [35]. This definition is supported by the understanding of the _common data environment_ (CDE) envisaged in the rail industry, as mentioned by Capgemini. This is regarded as the basis for the goal-oriented management of critical assets along their life cycle in rail transport [36]. In addition, IBM describes that a DT can represent not only the pure virtual representation of a physical object but also its entire life cycle and emphasizes in particular the interoperability of systems, i.e., the ability of different systems to work together as seamlessly as possible [37]. Several of the companies listed address the point that a DT obtains data from operations, i.e., through IoT sensors, and other internal and external sources of information.