Ontologies can be categorised. Ontohub's category system is maintained as an OWL ontology in Ontohub itself, see .
The backbone of Ontohub categories is The International Standard Classification of Education (ISCED). ISCED is a member of the United Nations International Family of Economic and Social Classifications and is the reference classification for organizing education programmes and related qualifications by levels and fields of education. The final draft of the recent ISCED version (June 2013) provides a hierarchy of educational domains. ISCED was selected among several other classifications, as the most appropriate reference classification that specifies knowledge-subjects, i.e. categories of knowledge-domains. The reference classification was extended with several categories which are not originally contained within ISCED, e.g. Space, Time, and Process.
During the ontology upload, it is advisable to select categories that are considered as the most relevant specification of an ontology-domain, i.e. the selected category should fit the domain of knowledge that the ontology in question represents. The criterion for selection of an appropriate category can be phrased as the questions:
- Does category <e.g. Space> fits the knowledge-domain that my ontology represents formally?
- Does my ontology belong to the domain of <category>?.
Accordingly, any ontology that represents mereological relations will fit to the category Space. Since an ontology can capture knowledge that intersects different fields, it is possible to select multiple categories for an ontology. An additional specification of categories will describe closer a particular ontology domain (e.g. EU-Biodiversity-Ontology covers domains: Space AND Biology AND Geography). Such a specification of categories facilitates findability and reusability of domain-specific ontologies.