.BioChem.Substances.BoundarySubstance .BioChem.Substances.BoundarySubstanceSubstance with a concentration not determined by reactions, i.e., the substance is on the boundary of the reaction system. The concentration of the substance can only be changed by events.
Corresponds to SBML species not changed by any SBML rules and with either or both of the boundaryCondition and fixed attributes set to true