## 2 Methodology  
In order to evaluate the use of BIM-based DT and XR technologies to improve maintenance operations, a systematic literature review methodology was used. The main aim is to observe how these technologies are used in the building industry for maintenance purposes and to identify the gaps in the literature. Then, this research can be used to provide future fields of research. Figure 1: Digital Model (**left**), Digital Shadow (**middle**), Digital Twin (**right**) [21]. Multiple databases were selected to perform this research:  
* ScienceDirect ([[https://www.sciencedirect.com/](https://www.sciencedirect.com/)]([https://www.sciencedirect.com/](https://www.sciencedirect.com/))) (accessed on 5 February 2021)
* Google Scholar ([[https://scholar.google.fr/](https://scholar.google.fr/)]([https://scholar.google.fr/](https://scholar.google.fr/))) (accessed on 5 February 2021)
* WebOfKnowledge ([[https://www.webofknowledge.com/](https://www.webofknowledge.com/)]([https://www.webofknowledge.com/](https://www.webofknowledge.com/))) (accessed on 5 February 2021)
* Scopus ([[https://www.scopus.com/](https://www.scopus.com/)]([https://www.scopus.com/](https://www.scopus.com/))) (accessed on 5 February 2021)  
Moreover, a manual search of Grey Documentation was performed to avoid relevant papers available on the Internet and published by non-academic institutions. This research was performed using the software Harzing's Publish or Perish ([[https://harzing.com/resources/publish-or-perish](https://harzing.com/resources/publish-or-perish)]([https://harzing.com/resources/publish-or-perish](https://harzing.com/resources/publish-or-perish)) (accessed on 5 February 2021)), which helps to perform a cross-database research. The reference manager software utilised was Zotero ([[https://www.zotero.org/](https://www.zotero.org/)]([https://www.zotero.org/](https://www.zotero.org/)) (accessed on 5 February 2021)) due to its large community and the automatic citation add-in for Microsoft Word. We decided to focus the research on the combined use of the terms \"_Maintenance_\", \"_BIM_\", \"_Extended Reality OR XR_\" and \"_Digital Twin_\". This research was conducted over a time window from 2005 to February 5, 2021, to observe the most recent developments and implementation of these technologies. A Venn diagram of the terms combinations has been created (see Figure 2). The size of the different coloured areas does not correspond to the number of items found for each combination but helps to identify the different combinations used. The different numbers are used to show the various combinations used: 1 for \"_Maintenance_\", 2 for \"_BIM_\", 3 for \"_Extended Reality OR XR_\" and 4 for \"_Digital Twin_\" However, to avoid missing any relevant paper, the term \"_Extended Reality OR XR_\" was broken down in its various subtypes: \"_Augmented Reality OR AR_\"; \"_Mixed Reality OR MR_\" and \"_Virtual Reality OR VR_\". Figure 2: Venn diagram of the terms combinations used for our research. The red outline delimits the combinations selected for our research. The red numbers represent the number of papers found for the combination of three and four terms. In the selected period, many papers were published for some of these combinations (e.g., More than 1200 only for the combination \"_XR AND Maintenance_\"). On the other hand, some of these combinations very few results (e.g., 5 results on the overall combination \"_BIM AND Digital Twin AND XR AND Maintenance_\"). The research was therefore extended to the papers using a DT based on a BIM for maintenance purposes and to the DT using XR devices for maintenance purposes. The combinations used for this research are outlined in red in Figure 2:  
* _\"BIM\" AND \"Digital Twin\" AND Maintenance_ (combination \"_124_\" in Figure 2). This combination represents 42 papers. * _\"Digital Twin\" AND (\"Extended Reality\" OR XR) AND Maintenance_ (combination \"_134_\" in Figure 2). This combination represents 30 papers.