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.