Elucidation: A realizable entity that (1) exists because the bearer is in some special physical, social, or institutional set of circumstances in which the bearer does not have to be, and (2) is not such that, if this realizable entity ceases to exist, then the physical make-up of the bearer is thereby changed. A role is thus always optional. Example of usage: the role of subject in a clinical trial; the role of a boundary to demarcate two neighboring administrative territories; the role of a building in serving as a military target; the priest role. Source: Robert Arp, Barry Smith, and Andrew D. Spear: Building Ontologies with Basic Formal Ontology. The MIT Press Cambridge, Massachusetts London, England. ©2015 Massachusetts Institute of Technology. ISBN 978-0-262-52781-1, Glossary. https://www.ebi.ac.uk/ols4/ontologies/bfo/classes/http%253A%252F%252Fpurl.obolibrary.org%252Fobo%252FBFO_0000023