## 2 Relevant Literature  
### Concept Definition  
One of the first documented examples of a DT was as a computer-aided design (CAD) object, which was an element in a product lifecycle management (PLM) process and its support systems [8]. This illustrates the fact that DT is a concept and not a specific technology. [PERSON] and [PERSON] [9] (p. 3) assert that \"a digital twin can be defined, fundamentally, as an evolving digital profile of the historical and current behavior of a physical object or process that helps optimise business performance\", and [PERSON] and [PERSON] [8] (p. 92) suggest that a DT \"is based on the idea that a digital informational construct about a physical system could be created as an entity on its own\". [PERSON] and [PERSON] [7] reviewed 46 digital twin definitions evident in the extant literature and suggested that DT can be defined as \"a virtual representation of a physical system (and its associated environment and processes) that is updated through the exchange of information between the physical and virtual systems\" (p. 2). [PERSON] [10] (para. 2) observes \"digital twinning is the process of creating a highly realistic model of a device, system, process or product to use for development, testing and validation\".  
The two-way exchange of information between the virtual and physical systems is a key aspect of a DT that differentiates it from most other technology concepts. A decade ago, [PERSON] [11] set out the three main components of a DT: products in the physical space, products in the virtual space, and the connections of data and information that unify both spaces. More recently, [PERSON] et al. [12] (p. 761) similarly noted that \"a digital twin is a virtual dynamic representation of a physical system, which is connected to it over the entire lifecycle for bidirectional data exchange\".  
Some authors also distinguish different types of DT projects. [PERSON] [13] (para. 8), for example, identifies three types: first, pre-built twins that \"come complete with products they represent or in generic form in business software\"; second, commercial--off the shelf--twins that can \"be started from templates\"; and third, \"custom twins\" that are \"custom developed from scratch\". [PERSON] notes \"custom twins are the easiest to modify but take the most time and skill\", whilst \"pre-built twins are the opposite\" and \"templates occupy the middle ground, are widely available, but rarely match the completeness of product digital twins\". In a similar vein, [PERSON] and [PERSON] [7] (p. 10) conclude \"several approaches can be considered: build a Digital Twin using existing commercial tools, construct a Digital Twin from commercial components, or a hybrid approach\".  
There are many areas of business in which DTs are being applied, but it is in manufacturing, and particularly in product design and development, that DTs are in greatest use. A DT can connect with different data flows, thus reflecting how a product can be better designed, manufactured, operated, and maintained. The insights gained on how the product changes in different conditions can contribute to innovation in any of the stages of product development [14]. [PERSON] et al. [6] (p. 1) affirm that \"the DT can evaluate the production decisions, access the product performance, command and reconfigure machines remotely, handle the troubleshoot equipment remotely and connect systems/processes to improve monitoring and optimise their control\", and that \"DTs can also be applied for process control, process monitoring, predictive maintenance, operator training, product development, decision support, real-time analytics and behavior simulation\".