According to the mmCIF dictionary, ; The value of _atom_site.id must uniquely identify a record in the ATOM_SITE list. Thus it would not be permitted to use the same _atom_site.id for the 'same' atom each time it occurs in different models. For the 'same' atom in different models, all the identification fields (atom name, residue name, chain, etc.) are identical; the only difference is in the atom number and, of course, the coordinates. One could, of course, make a separate entry out of each model but that would become quite unwieldly with the large number (>>50) of models that are deposited for some structures.