## 2 Materials and Methods
Due to the enormous number of heterogeneous results, a qualitative approach complements the study. This is initiated by specifically searching for lifecycle models aiming at an overarching level (see Table 1, bottom part):  
* Search term vector TS 2.1: [lifecycle model, life cycle model]. * Search term vector TS 2.2: [product, system]. * Search term vector TS 2.3: [holistic, generic]. Even with that specialization, the results are just embedding specific research into a wider context without dedicated methodical approaches. Usage of generic/holistic product lifecycle terms can be recognized mostly in motivation and introduction, but then the focus \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Web of Knowledge**} & \multicolumn{4}{c}{**ScienceDirect**} \\  & **Search Term** & & **1945–2021** & \multicolumn{2}{c}{**Last 5 Years**} & \multicolumn{2}{c}{**Review Articles,**} & \multicolumn{2}{c}{**Review Articles**} \\  & & & & & & & **Book Chapters** & & & \\
**TS1.1** & **TS1.2** & **TS1.3** & **Total** & **Total** & **Highly Cited** & **1995–2021** & **Last** & **1995–2021** & **Last** \\  & product & \({}^{*}\) & 371 & 324 & 8 & 14,662 & 8170 & 2204 & 1222 \\ (lifecycle OR & & literature & 77 & 72 & 4 & 8751 & 5695 & 1516 & 1104 \\ \cline{2-10} “life cycle” & system & \({}^{*}\) & 321 & 276 & 8 & 18,273 & 9651 & 2604 & 1621 \\ AND & literature & 65 & 61 & 5 & 10,436 & 6476 & 1724 & 1215 \\ (circular) & plant & \({}^{*}\) & 61 & 52 & 1 & 9015 & 5128 & 1566 & 1054 \\ (engineering & OR & literature & 6 & 6 & 1 & 5601 & 3566 & 1124 & 821 \\ OR changing & asset & \({}^{*}\) & 17 & 15 & 0 & 2121 & 1337 & 278 & 209 \\ OR design & & literature & 3 & 3 & 0 & 1530 & 1069 & 234 & 180 \\ \cline{2-10}  & software & \({}^{*}\) & 25 & 21 & 0 & 7752 & 4630 & 689 & 499 \\  & literature & 6 & 6 & 0 & 4531 & 3125 & 556 & 419 \\ \hline
**TS2.1** & **TS2.2** & **TS2.3** & & & & & & & \\ \hline “lifecycle model” OR & product & holistic & 5 & 1 & 0 & 571 & 230 & 51 & 34 \\ “model” OR & generic & 7 & 2 & 0 & 1140 & 77 & 305 & 31 \\ “life cycle model” & system & holistic & 10 & 4 & 0 & 1284 & 85 & 341 & 33 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of quantitative literature analysis (numbers finally updated on 2 May 2021, asterisk means wildcard). is shifted to specific aspects, such as sensors in Industry 4.0 networks or modelling aspects in Model Based Systems Engineering. The results of this analysis are documented in the requirements cumulated from life-cycle models of all perspectives and in lifecycle model portfolios, covering the dimensions \"means of Product Creation\" (technical system, product-service system, further environment), \"way of cross-linking lifecycle phases\" (linear model, circle as a design element, circular model) and \"degree of detail\" (conceptual sequence, information flow, material flow). English results are validated by double-checking with corresponding German search terms. These foundations are reflected and generalized into an overarching perspective. The synthesis subsumes the following:  
* Integrating core elements of complementary models. * Contextualizing lifecycle phases regarding different perspectives. * Visualizing both sequential dependencies and circularity (focusing on data/knowledge and products/material). * Providing means for adaptation/tailoring. The gPLC model is validated by retrospective application to the three cases carried out by the authors and introduced in Chapter 1: The RepAIR case [25], the OptiAMix case [26] and the SugarFab case [27]. Characteristics are documented in Table 2.  
These cases are selected due to the complementary characteristics. First, they cover complementary domains. In aeronautics (RepAIR), products are engineered for a long period of product life with corresponding MRO demands. For the OptiAMix case, automotive was chosen out of several project case studies due to the relevancy as a B2C market with its outstanding dynamics.