## 4 Discussion  
Although different definitions of DT can be found in the analyzed literature, commonly used definitions can be identified. A comparison between these shows that the levels of detail, as well as the focus area, differ, but the basic concept remains the same. In comparison with [PERSON], [PERSON] et al. added two dimensions and described the dimensions in more detail. This is also the most often used definition within the analyzed literature. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Objective** & **Records** & **Records [\%]** \\ \hline monitoring & 5 & 25.00 \\ production scheduling & 3 & 15.00 \\ AGV control & 3 & 15.00 \\ overall equipment effectiveness (OEE) & 3 & 15.00 \\ line balance rate & 2 & 10.00 \\ reaction time & 2 & 10.00 \\ workstation design & 2 & 10.00 \\ lead time & 2 & 10.00 \\ per capita productivity & 1 & 5.00 \\ number of operators & 1 & 5.00 \\ warehouse costs & 1 & 5.00 \\ energy consumption & 1 & 5.00 \\ travel distance & 1 & 5.00 \\ travel time & 1 & 5.00 \\ shop-floor management & 1 & 5.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Targeted objectives. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Domain** & **Records** & **Records [\%]** \\ \hline laboratory production & 5 & 25.00 \\ not assignable & 4 & 20.00 \\ automotive & 3 & 15.00 \\ electronics & 2 & 10.00 \\ metalworking & 2 & 10.00 \\ distribution center & 1 & 5.00 \\ aerospace & 1 & 5.00 \\ warehouse & 1 & 5.00 \\ energy & 1 & 5.00 \\ others & 1 & 5.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Application areas. Therefore, the level of implementation of the found case studies is also evaluated on this basis. It turns out that only four papers present a fully implemented DT for the core activities of logistics [41; 42; 43; 46]. Two of these papers show implementation in laboratory environments, leaving two industrial applications. In general, most case studies can be assigned to laboratory production lines, followed by applications in the automotive, electronic, and metalworking industries. Besides the different levels of implementation, different approaches for the creation of a VM can also be observed. For production logistics processes, DES is most often used, as it is also the state-of-the-art for simulating production logistics systems. Some authors present predefined building blocks and standardized processes to speed up the creation of DES models. However, most available DES software solutions still lack real-time capabilities [35], which are an essential part of DT technology. For the identified PHM processes, physical simulations are used. Some hybrid simulation approaches can also be found to integrate control mechanisms of decentralized applications. To successfully integrate a DT, a connection to enterprise information systems, as well as to sensors and actuators must be established. Different information systems can be found in various configurations in companies. The connection between and consistency of data in these systems can also vary widely. Another difference in the analyzed DTs is their main objective. Consequently, different requirements must be met, and individual consideration must be given to the information systems to which a bidirectional connection must be established. It is also conceivable that DT technologies will become an inherent part of enterprise information systems or manufacturing execution systems. To highlight the contribution of this literature analysis, a comparison is made with existing literature reviews related to DT. [PERSON] et al. analyzed papers regarding DT in the manufacturing area. In this context, a DT was described as consisting of a digital shadow, a digital model, and a bidirectional link between these two elements. Based on this description, the level of integration of the analyzed literature was evaluated. A focus area and the core technologies of these papers are also described [11]. [PERSON] and [PERSON] analyzed 19 different definitions of DT.