All the classes that represent a conceptual category
Barry Smith et al. A Strategy for Improving and Integrating Biomedical Ontologies. AMIA 2005
By endorsing to a given Upper Level Ontology when building a domain ontology the ontologists makes the integration of the ontology with other ontologies a much easier process. Besides, the ontology becomes a cleaner model with different modules
Different ontologies of a given domain share very general types of concepts, like Substance, Modifier, etc. These types of concepts are grounded in philosophical criteria, like distinctions between Occurents and Continuants. The different domain ontologies can thus be integrated in one Upper Level Ontology, each ontology having different relationships pointing to the concepts of the Upper Level Ontology. The Upper Level Ontology used here as an example is the Ontology of Biomedical Reality (OBR)
Foundational ontology
Good Practice
The different hierarchies of primitive classes must be asserted using disjoints
The ontology is committed to a concrete view of the knowledge domain (given by the Upper Level Ontology), and therefore the use and implantation of Upper Level Ontologies is very controversial
To create an ontology that can integrate different ontologies in itself
Upper Level Ontology