## 2 Research Methodology  
This research followed the SLR as a basic scientific activity that delivers a clear and comprehensive overview compared to descriptive literature reviews. The formation of a basic framework for an in-depth analysis and a scientific process can be possible by using this SLR. The systematic literature followed a sequence of five steps, as mentioned in [10], which are as follows. 1. Formation of questions;
2. Finding the studies;
3. Study preference and evaluation;
4. Investigation and combination;
5. Reporting and using the results. Step 1. Formation of questions:  
Research Question 1. What is the role of degradation, residual life distribution, workload strategy, upgradation, and predictive maintenance on flexible unit systems? Research Question 2. How to integrate the degradation and upgradation models to the flexible unit systems? Step 2. Finding the studies:  
This step concerns how to find and choose the bibliographic database or search engine, and additionally the search strings. The research questions have been considered in this search for literature reviews. Following similar literature reviews [14; 15; 16] and three bibliographic databases, i.e., Web of Science, Scopus, and Science Direct, a remarkable quantity of published literature on degradation rate, residual life distribution, workload strategy, upgradation, and predictive maintenance, including very relevant and important journals in this area, has been considered. Additionally, also considered were advanced analytics, like descriptive, predictive, and prescriptive ones, to analyze the machine's historical data for improving the efficiency of the system. Tables 1-3 show the search strings searched in the data bases and the results obtained using the three mentioned databases. However, sorting the selected research articles and selecting the publication title between 2009-2020 shows 603 articles for the search string \"Flexible unit systems\" (or) \"Flexible machine systems\" and \"Degradation\" (or) \"Degradation rate\", 167 articles for the search string \"Flexible unit systems\" (or) \"Flexible machine systems\" and \"Residual Life Distribution\" (or) \"Residual life\", 140 articles for the search string \"Flexible unit systems\" (or) \"Flexible machine systems\" and \"workload strategy\" (or) \"workload adjustment\", 104 articles for the search string \"Flexible unit systems\" (or) \"Flexible machine systems\" and \"Upgradation\", and 243 articles for the search string \"Flexible unit systems\" (or) \"Flexible machine systems\" and \"Predictive Maintenance\", respectively. Step 3. Study preference and Evaluation:  
In this step, filtering criteria were explicated, to choose only relevant studies to add in the review, in which the studies actually addressed the research questions. From 1995 to 2008, articles were excluded because they were just consigned to the small percentage of the examples. 11 years (2009-2020) of related studies were performed to focus on recent studies, methodologies, and technologies. The article journals of document type were sorted from the search results and the best articles distributed in peer-reviewed journals in English were contemplated. [PERSON] et al. (2017) argue that restricting the search to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Search String** & **Search Field** & **Date of Search** & **No. of Results** \\ \hline “Flexible unit systems” (or) “Flexible machine systems” and “Degradation” (or) “Degradation Rate” & Topic & 11 August 2020 & 273 \\ \hline “Flexible unit systems” (or) “Flexible machine systems” and “Residual Life” (or) “Residual Life Distribution” & Topic & 11 August 2020 & 34 \\ \hline “Flexible unit systems” (or) “Flexible machine systems” and “Workload strategy” (or) “Workload adjustment” & Topic & 11 August 2020 & 42 \\ \hline “Flexible unit systems” (or) “Flexible machine systems” and “Predictive Maintenance” & Topic & 11 August 2020 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Search string and number of results from Web of Science.