## 7 Discussion and Challenges
### Future Usage: DT Interaction and Enhanced Maintenance Operations  
In their proposed interaction framework based on XR technologies in DT, [PERSON] et al. [75] have proposed a scheme to show the difference in interaction performance between VR, AR and MR. Their work has inspired us to create our own interaction scheme between the user, the DT and the application framework during maintenance operations (see Figure 16). The scheme is composed as follows:  
Figure 14: Type of paper for each type of DT.  
Figure 15: Main usage domains of the different DT types.  
* The Digital Twin is represented by three parts:
* _Physical part_: This represents the physical asset of the DT (e.g., equipment, building or equipment). Data can be sent from and to this part thanks to the _data processing_ part. However, if on-site, the _User_ can also interact directly with it, especially during maintenance operations or visual inspections.
* _Digital part_: This is the digital representation of the asset, with the semantic and real-time data gathered with smart sensors and processed by the _data processing_ part. The data can then be linked to the 3D representation of the asset and then displayed to the user once processed in the appropriate language.
* _Data processing_: This contains all the communication protocols allowing exchanges between physical and digital parts, and the algorithms that process the raw data before sending it to the digital part. It also contains the decision and prediction algorithms as well as the ones allowing the transmission of the user's commands to the digital part to display information and to the physical part for the control of the real system.
* _HMI_: The human-machine interface (HMI) part represents the communication interface between the user/expert and the DT (e.g., Information displayed and equipment's controls). This interface can either represent an XR device or a classic computer software allowing 3D information to be visualised thanks to the _data processing_ part that translates the commands into the appropriate language. It can also represent VR training applications using BIM-based DT to train the user on realistic situations through an immersive environment and collected data from real-life situations [78; 89].
* _User/Expert_: The on-site user can interact with the DT thanks to both the _HMI_ part and direct interaction with the _Physical_ part, especially during maintenance operations. On the other hand, the remote expert can interact with the DT only by using an _HMI_, as the distance prevents any direct interaction with the _Physical_ part. The blue two-way arrow represents the collaboration that can happes between the on-site user and the remote expert when needed, either using the _HMI_ part (such as a specific MR application) or with an external device (such as a phone call).
* _[PERSON]_ The lack of use cases has already been observed by some authors [28; 29; 33; 34; 51; 78]. Various factors can explain this difficulty. In addition to technical challenges, such as data management and issues related to the use of XR devices, multiple authors have observed  
Figure 16: Interactions for an on-site user and a remote expert.  
that changes are needed in the organisation and management of maintenance procedures for a correct implementation (see Table 9).