## 3 Results  
### Descriptive Analyses  
Table 1 shows the result of the rating process of the identified records. Of these, 24 papers were classified as high-appropriate, 23 as medium-appropriate, and 46 as low-appropriate. As many as 21 of the high-appropriate papers were available to the authors and, therefore, included in the further analyses. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Appropriateness** & **Records** & **Records [\%]** \\ \hline Total & 93 & 100.00 \\ High-appropriate papers & 24 & 25.81 \\ Medium-appropriate papers & 23 & 24.73 \\ Low-appropriate papers & 46 & 49.46 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Rating of the screened records. Figure 1: Sankey-based visualization of the PRISMA selection process [25; 26]. Out of the screened records, no papers were published in 2015 and 2016. Five papers (5.38%) were published in 2017, eight papers (8.60%) in 2018, 27 papers (29.03%) in 2019, 42 papers (45.26%) in 2020, and 21 papers (22.58%) in 2021, up to the date of conducting the literature search. Publications from 2015 to 2020 are shown in Figure 2, indicating an increasing research interest in this topic. Most often, papers within the defined limitations are published in Applied Sciences, Journal of Manufacturing Systems, Sensors, Academy of Strategic Management Journal, IEEE Access, IFAAC Papersonline, and Robotics and Computer Integrated Manufacturing, as noted in Table 2.  
To identify research collaborations, the number of authors per publication was analyzed. The allocation of the number of authors is shown in Figure 3. A majority of 22 papers (23.66%) were written by four authors; 20 papers (21.53%) were written by three authors, 15 papers (16,13%) by two authors, eleven papers (11.83%) by six authors, 10 papers (10.75%) by five authors, six papers (6.45%) by one author, six papers by seven authors, two papers (2.15%) by eight authors, and one paper (1.08%) by nine authors. When directly searching for the term DT, the Scopus database identifies [PERSON] as the author with the most records (41) in this general field. However, when searching for DT in production logistics in detail, [PERSON] leads the list of authors with six records. All authors with at least three records in the area of interest are displayed in Figure 4. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Source** & **Records** & **Records [\%]** \\ \hline Applied Sciences Switzerland & 5 & 5.38 \\ Journal of Manufacturing Systems & 4 & 4.30 \\ Sensors & 4 & 4.30 \\ Academy of Strategic Management Journal & 3 & 3.23 \\ IEEE Access & 3 & 3.23 \\ IFAC Papersonline & 3 & 3.23 \\ Robotics and Computer Integrated Manufacturing & 3 & 3.23 \\ EAI Endorsed Transactions on Energy Web & 2 & 2.15 \\ International Journal of Computer Integrated Manufacturing & 2 & 2.15 \\ Procedia CIRP & 2 & 2.15 \\ Others & 62 & 66.67 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Publication sources. Figure 2: Identified records by year of publication. Figure 5 shows a heat map of the number of records by country to which the authors are assigned. In addition, Table 3 presents the countries to which at least three records are assigned. Most authors in this area are assigned to Germany (26), followed by China (14), the Russian Federation (9), and Hong Kong (8). Figure 4: Records by author. Figure 5: Heatmap of records by country. Figure 3: Research collaborations. Although most of the authors can be attributed to Germany, they have varying affiliations. In the present case, most authors are affiliated with the University of Hong Kong (7), followed by the University of Jinan (6) in China. The institutions to which at least three records can be linked are listed in Table 4.  
Index keywords and author keywords were analyzed, and the most used keywords are listed in Table 5. As expected, the keyword _Digital Twin_ is most often used (index 56.99%, author 70.97%). Due to the described similarities to _CPSs_, it is unsurprising that this keyword is also part of the 10 most used keywords (index 16.13%, author 10.75%).