## 3 Results
### Case Study Analyses  
The analyzed papers describe different concepts and approaches to speed up the creation of the VM, connections to different enterprise information systems, successful linkages to and implementation of the DT, as well as the varied potential of this technology. Nevertheless, different implementation levels of the described DT for production logistics processes were found. To gain further insights into the state of the art, papers reporting case studies (20 papers) are analyzed in more detail. As displayed in Figure 5, the five dimensions of [PERSON] et al. are most often used in the studies found to define a DT. Therefore, the authors also apply these dimensions to evaluate the degree of coverage of the characteristics of a DT in the presented case studies in Table 9.  
The distribution of the fulfillment rate can be seen in Figure 9. A majority of 18 papers (90%) present a VM of a real physical system or entity. DT data were considered by 14 papers (70%). A service system is implemented by twelve papers (60%) and a connection between the other dimensions was created by seven (35%) papers. These seven papers (35%) consequently implemented all five dimensions, with two of them (10%) created for laboratory environments, while three other papers (15%) deal with PHM, which is not consistent with the core logistics tasks. Figure 9: Distribution of fulfillment rate. Also, different VMs were presented to mirror the physical entity. The decision on which model should be implemented depends on various factors. Also, a multitude of modeling technologies (e.g., discrete event simulation, physical simulation, automated guided vehicle simulation, agent-based simulation) were found by systematically analyzing the resulting research records. Table 10 summarizes the identified modeling technologies for production logistics processes. In general, DES is the most used simulation technique for production and logistics systems [51]. This is consistent with the results of this analysis, showing that DES is also most often used to create a VM in the context of DT for production logistics processes (nine papers). Two papers propose a combination of DES and AGV simulators to integrate the control mechanism of the AGV. An increasing trend toward such hybrid simulation approaches can be observed regarding simulations in the context of Industry 4.0 [52]. Regarding PHM, three papers use physical simulations. Several other approaches were identified, but their application is indicated to be quite low because each of them was only referenced once. In the analyzed papers, various objectives were pursued to tackle different problems. To give an overview of possible potentials for logistics processes, Table 11 shows the objectives addressed by the presented studies. In general, the purpose of a VM is to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Abbr.** & **Name** & **Records** & **Records [\%]** \\ \hline DES & Discrete Event & 9 & 45.00 \\ PS & Simulation & 3 & 15.00 \\ AGV Simulator & Automated Guided & 2 & 10.00 \\  & Vehicle Simulation & 1 & 5.00 \\ ABS & Agent-based & 1 & 5.00 \\  & Simulation & 1 & 5.00 \\ AM & Analytical Model & 1 & 5.00 \\ CRS & Catmull-Rom Spline & 1 & 5.00 \\ GE & Game Engine & 1 & 5.00 \\ KD & Knowledge Database & 1 & 5.00 \\ MDO & Multidisciplinary & 1 & 5.00 \\  & Design Optimization & & \\  & Time-Weighted & & \\ TWMLR & Multiple Linear & 1 & 5.00 \\ Regression & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Virtual Models for Digital Twins.