The component identifier in the second of the two entities containing the linkage.
               For polymer entities, this data item is a pointer to _pdbx_reference_entity_poly_seq.mon_id
               in the PDBX_REFERENCE_ENTITY_POLY_SEQ category.
               For non-polymer entities, this data item is a pointer to
               _pdbx_reference_entity_nonpoly.chem_comp_id in the
               PDBX_REFERENCE_ENTITY_NONPOLY category.