A cellular location, e.g. 'cytoplasm'. This should reference a term in the Gene Ontology Cellular Component ontology. The location referred to by this property should be as specific as is known. If an interaction is known to occur in multiple locations, separate interactions (and physicalEntities) must be created for each different location. If the location of a participant in a complex is unspecified, it may be assumed to be the same location as that of the complex. A molecule in two different cellular locations are considered two different physical entities.
Source:http://www.biopax.org/release/biopax-level3.owl#cellularLocation