              The value of _atom_site.id must uniquely identify a record in the\
               ATOM_SITE list.\
\
               Note that this item need not be a number; it can be any unique\
               identifier.\
\
               This data item was introduced to facilitate compatibility between\
               small molecule and macromolecular files. In the small molecule\
               files, _atom_site_label is the identifier for the atom. In the\
               macromolecular files, the atom identifier is the aggregate of\
               _atom_site.label_alt_id, _atom_site.label_asym_id,\
               _atom_site.label_atom_id, _atom_site.label_comp_id and\
               _atom_site.label_seq_id. For the two types of files to be\
               compatible, a formal identifier for the category had to be\
               introduced that was independent of the different modes of\
               identifying atoms. For compatibility with older files,\
               _atom_site_label is aliased to _atom_site.id.