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 physicalEntityParticipants) must be created for each different location. Note: If a location is unknown then the GO term for 'cellular component unknown' (GO:0008372) should be used in the LOCATION property. 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. In case of conflicting information, the location of the most outer layer of any nesting should be considered correct. Note: Cellular location describes a specific location of a physical entity as it would be used in e.g. a transport reaction. It does not describe all of the possible locations that the physical entity could be in the cell.
Source:http://www.biopax.org/release/biopax-level2.owl#CELLULAR-LOCATION