Definition: Any additional special characteristics of a physical entity in the context of an interaction or complex. These currently include stoichiometric coefficient and cellular location, but this list may be expanded in later levels. Comment: PhysicalEntityParticipants should not be used in multiple interaction or complex instances. Instead, each interaction and complex should reference its own unique set of physicalEntityParticipants. The reason for this is that a user may add new information about a physicalEntityParticipant for one interaction or complex, such as the presence of a previously unknown post-translational modification, and unwittingly invalidate the physicalEntityParticipant for the other interactions or complexes that make use of it. Example: In the interaction describing the transport of L-arginine into the cytoplasm in E. coli, the LEFT property in the interaction would be filled with an instance of physicalEntityParticipant that specified the location of L-arginine as periplasm and the stoichiometric coefficient as one.
Source:http://www.biopax.org/release/biopax-level2.owl#physicalEntityParticipant