# DTAG: A Methodology for Aggregating Digital Twins Using the WOTDT Ontology
## 2 State of the Art  
The concept of digital twin was introduced by [PERSON] as a _product lifecycle management_ (PLM) [7] for NASA in the aerospace field, defining it as a digital representation of a physical object or system that can be used to analyse its behaviour and effectiveness. The proposal models a three-dimensional architecture: the physical dimension, which represents the actual asset in the real world; the virtual dimension, which represents the software representation of the physical asset; connection parts, which exist between the physical and virtual dimensions. Following the three-dimensional methodology, the researchers focused on refining aspects of these dimensions to address new challenges [3; 11; 12; 13]. For instance, NASA [12] enhanced the current simulations conducted in the virtual dimension with the aim of better managing US Air Force aircraft over their operational lifetime through the development of customised structural management strategies and by integrating real-time data collection into the physical dimension. **Five-dimensional model approach:** Subsequently, in 2017, [PERSON] and [PERSON] [8] proposed a five-dimensional model of a DTw, extended from the three-dimensional approach. In this proposal, the physical dimension is referred to as the physical entity dimension, while the connections are termed the digital twin connection dimension. Instead, the virtual dimension is divided into three dimensions: the virtual entity dimension, which includes the description of the DTw models (such as rules, behavioural, physical, etc. ), the data dimension of the digital twin, where all the data utilised by the DTw are stored, and the digital twin service dimension, which includes all the services used by the DTw. **DTw levels:** Based on the five-dimensional model, three-level differentiation of DTws was proposed [14]: (i) unit level, which is the minimum unit that can be represented as a DTw (e.g., a robotic arm belonging to an assembly line); (ii) system level, it is considered as the integration of multiple DTws at unit level that cooperate with each other (e.g., the set of robotic arms belonging to the assembly line); (iii) system of systems level, which is the set of system-level DTws that need each other to perform more complex operations (e.g., the set of assembly lines in the factory where the assembly process is being carried out). This model of DTws divided into levels is also presented in other articles [2], where the existence of a world of interconnected twins is proposed, where the DTw of a system can be the composition of a system made up of interconnected DTws of a lower level. **DTw aggregation:** [PERSON] and [PERSON] presented the idea of DTws aggregation in 2016 [3], outlining concepts such as a DTw prototype (DTwP), a DTw instance (DTwI), a DTw aggregate (DTwA), and a DTw environment (DTwE). A DTwP characterises a physical object to the extent that an object can be produced using the details provided in the DTwP. A DTwI represents an instance of a DTwP, which is linked to a specific physical object and persistently collects data about this object during its lifecycle. The DTwA aggregates all DTwIs of a specific category to create a larger and more complete dataset concerning the operation of a type of physical object. Finally, the DTwE serves as an integrated interdisciplinary physics application environment that uses the DTws for multiple purposes, such as the simulation of future system behaviour. [PERSON] et al. [15] used aggregation to collect, process, and reduce data from multiple DTws to inform a control application. [PERSON] et al. [16] aggregated historical data (collected within DTws) to handle large amounts of historical data. Similarly, [PERSON] et al. [17] employed hierarchical data format (HDF) data compression to aggregate large amounts of heterogeneous data, providing a more comprehensive and integrated data representation. [PERSON] [18] employed personal DTws to maintain patient medical records.