Whenever you introduce an ontology in a case, you create an instance, or: an occurrence, of that ontology.
During configuration, you have defined the ontology 'Person'.
When a user gets ready to create a case for potential client Peter Müller, they will select 'Person' from available ontologies. From that moment on, 'Peter Müller' is an instance of the ontology 'Person'.
As the case moves through the process, atfinity will include all the information fields, roles, documents and proofs you have defined during the configuration for the ontology 'Person'.