# Envisioning Digital Practices in the Metaverse: A Methodological Perspective +
## 4 Designing and Coordinating Digital Entities
### Digital Twins  
Digital twins represent a two-way flow of data between a physical object and its digital counterpart. Changes made to the physical object are automatically reflected in the digital twin and vice versa [9]. Digital twins have become a widely used concept in advanced manufacturing and networked contexts [4], allowing for remote monitoring and control with advanced AI techniques [4]. The ability of the digital twins to provide synchronised logical counterparts of physical entities has been leveraged to engineering self-organising cyber-physical systems (CPSs) [22]. This virtual representation of physical entities is incredibly valuable in the digital transformation of our society [6], aiding development in smart cities and modern healthcare. By facilitating information exchange between physical and virtual systems, digital twins enable the prediction and optimisation of complex CPSs, making them an essential software engineering paradigm [7]. Digital twins are now being considered as software entities that follow the entire lifecycle of their physical counterparts, which requires systematic DT engineering and management [23]. Recent works discuss state-of-the-art methodologies for designing and implementing digital twins, considering various phases, such as requirements definition, architectural planning, integration and verification, existing protocols and standards [24]. As an example, in [23], the authors introduce a DT lifecycle model, named the \(FA^{3}ST\) approach, by putting forth methodologies and tools for effective DT management by contextualising their proposal within the framework of Industry 4.0. Managing the relationships between all the embedded entities can be overwhelming when dealing with dynamic environments. Our scenario, on the other hand, involves multiple stakeholders with diverse behaviours, resulting in numerous interactions within the virtual/physical space. A single digital twin with a massive model would be too domain-specific, making it challenging to reuse in different circumstances. Conversely, adopting a fine-grained approach in which everything (that is relevant) is modelled through a different digital twin was shown to be successful in designing complex scenarios like smart cities [25]. Our design approach is based on a detailed level of granularity. It allows us to model customers, employees, and resources in the shop as different digital twins in our scenario. The system's digital twins are used to represent various physical spaces. Three digital twins are dedicated to different areas of the grocery shop. The first digital twin provides a real-time description of the layout of the entire building. Separate digital twins are created for each aisle, shelf, cash desk, and customer assistant desk. For instance, the shelf digital twin reflects the real layout of products, price signs, and nearby people. The product inventory is also a digital twin that shows the availability of products in the shop. A specific case of digital twins is the human digital twin, representing a copy (or counterpart in cyberspace) of a real person [26]. The concept has been raised in socio-technical systems to put humans into the loop. In medicine, for example, human digital twin models the fact that \"the physical system is the patient, or a particular organ or physiological system to be managed\" [27]. In surgery, having an updated model of the patient's body could be of capital importance for the surgeon before and during an operation. In MR applications, human digital twins allow alignment of the avatar to the user, reflecting the user's mood, interests, and behaviour. In our example, human digital twins represent employees, mirroring their competencies and roles within the market. Finally, digital twins also represent in-person and virtual customers, reflecting their habits, preferences, and behaviour. Now, the task at hand is to effectively merge these multiple digital twins in a collaborative manner.