The HDT ontology is an extension of CIDOC CRM and follows the naming conventions that have been applied throughout the CIDOC CRM. All the new declared classes and properties were given both a name and an identifier constructed according to the conventions used in the CRM base model. For classes, that identifier consists of the letters HC (Heritage Class) followed by a number. Properties were also given a name and an identifier, constructed according to the same conventions. That identifier consists of the letters HP (Heritage Property), followed by a number, which in turn is followed by the letter “i” every time the property is mentioned “backwards”, i.e., from target to domain (inverse link). “HC” and “HP” do not have any other meaning. They correspond respectively to letters “E” and “P” in the CRMbase naming conventions, where “E” originally meant “entity” (although the CRMbase “entities” are now consistently called “classes”), and “P” means “property”. Whenever CRMbase classes are used in our model, they are named by the name they have in the original CRMbase. The same applies for all the extensions which also follow the same conventions.
The most general notion of the model, its focal concept, is HC1 Heritage Entity, which denotes anything of the real world regarded as valuable because of the contribution to society, knowledge and culture. In that sense, what distinguishes this general concept from the others is the value that acquires (a functional characteristic) some entity relative to the society and culture. This characteristic is a dynamic, contextual attribute that is produced by the society. As such, it constitutes a role, i.e., in the sense of the DOLCE ontology2, it does not carry a condition for identifying an instance to be distinct from others and to be diachronically the same or not. This means that the item characterized as Heritage Entity exists independently; its nature and identity does not change or depend on the heritage (social) context.
HC1 serves as an entry point for linking to HC2.
HC2 Heritage Digital Twin concept constitutes an epistemic consideration which may vary in point of view and scale between different authors. Updating an instance of HC2 does not make the previous disappear, as with all Conceptual Objects. Therefore, an instance of HC2 should be identified by its propositional content and creating Actor. If uniqueness is required, it constitutes an attribute assigned to one instance by and relative to a particular Actor, typically even a project. In that sense, there are no other objective criteria to distinguish a digital twin from another. Therefore, contents of different instances may include or overlap each other. Therefore, rather its contents and not the instance itself constitutes robust concepts that can be shared in a discourse and are unambiguous across contexts. In other words, the notion of a Heritage Digital Twin instantiated by a node in a semantic network makes only sense with respect to a particular functional role in a project context, representing facts that an information provider has agreed to provide digital material about.