# DTAG: A Methodology for Aggregating Digital Twins Using the WOTDT Ontology
## 3 WoTDT Ontology Development
### WoTDT Requirements  
The specification of ontology requirements is derived from the analysis of ontologies from the literature, the five-dimensional model approach, and the aggregation method proposed in this article, which was refined during its adoption and use in the COGITO H2020 project. In contrast to the ontologies found in the literature that are limited to the virtual aspects of a DTw, WoTDT ontology outlines the necessary requirements to conceptualise the five dimensions, as defined by [PERSON] and [PERSON] [8]. Similarly, the virtual component discussed in WoTDT presents similarities to the ontologies described in the existing literature. From these inputs, a collection of ontological requirements was generated in the form of competency questions and sentences in natural language. Table 1 shows the new requirements obtained from the specification process.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**ID** & **Competency Question/Statement—Possible Answer** \\ \hline WOTDT-1 & A digital twin is a thing. \\ \hline WOTDT-2 & A digital twin contains five dimensions. \\ \hline WOTDT-3 & Physical entity is a dimension that represents the real-world asset of the digital twin. \\ \hline WOTDT-4 & Digital entity is a dimension that represents the digital asset of the digital twin. Also, it contains the dimensions of the virtual entity, DTw data, and DTw services. \\ \hline WOTDT-5 & Virtual entity is a dimension that represents the different models used in the digital twin. \\ \hline WOTDT-6 & DTw data is a dimension where are stored all the data used in the digital twin. \\ \hline WOTDT-7 & Digital twin services is a dimension in which all services of the digital twin are described. \\ \hline WOTDT-8 & DTw connection is a dimension in which all the connections between other dimensions in the DTw and with other external DTw dimensions are described. \\ \hline WOTDT-9 & Virtual entity dimension can have models. \\ \hline WOTDT-10 & Which kind of models can be described in the virtual entity dimension? The models can be from rules, behavioral, physical and geometric models to semantic models like ontologies. \\ \hline WOTDT-11 & DTw data dimension can have resources that can be used to represent different types of data stored at the digital twin. \\ \hline WOTDT-12 & DTw service dimension can have interaction affordances from the WoT thing descriptions ontology to represent the different services used at the digital twin. \\ \hline WOTDT-13 & DTw connection dimension can have different connections. \\ \hline WOTDT-14 & Which type of connections can the digital twin connection dimension describe? The connections defined in the DTw connection dimension are described with the different existing elements of other dimensions of the DTw, such as models, resources, and interaction affordances, and the connections with external things such as other DTws. \\ \hline WOTDT-15 & Digital twin instance (DTwI) describes a specific corresponding physical product to which an individual DTw remains linked throughout the life of that physical product. \\ \hline WOTDT-16 & Digital twin aggregate (DTwA) describes the aggregation of DTwI and DTwA. Unlike the DTwI, the DTwA may not be an independent data structure. It may be a computing construct that has access to all DTwIs and queries them either ad-hoc or proactively. \\ \hline WOTDT-17 & Connections between DTws can be represented as external connection points. \\ \hline WOTDT-18 & Aggregations between data models metadata or the data resources metadata of the DTws can be represented in the resulting DTwA within the connections between them. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Requirements for WoTDT ontology.