Ontologies are formal representations of knowledge. They consist of terms (formally defined as classes) and relationships between classes. They are constucted to be both human-readable and computer-parsable.
If you are new to ontologies, take a look at the Shirt Ontology Tutorial. It explains the structure of the content of the Foundational Model of Anatomy (FMA). This is the ontology that the human canonical anatomy content of the OCDM is based on. This tutorial should help to demystify the difference between the “regional part” and “constitutional part” relationships.