An alternative identifier for _atom_site.label_seq_id that may be provided by an author in order to match the identification used in the publication that describes the structure. Note that this is not necessarily a number, that the values do not have to be positive, and that the value does not have to correspond to the value of _atom_site.label_seq_id. The value of _atom_site.label_seq_id is required to be a sequential list of positive integers. The author may assign values to _atom_site.auth_seq_id in any desired way. For instance, the values may be used to relate this structure to a numbering scheme in a homologous structure, including sequence gaps or insertion codes. Alternatively, a scheme may be used for a truncated polymer that maintains the numbering scheme of the full length polymer. In all cases, the scheme used here must match the scheme used in the publication that describes the structure.