A code which identifies a cluster of atoms that show long-range
               positional disorder but are locally ordered. Within each such
               cluster of atoms, _atom_site.disorder_group is used to identify
               the sites that are simultaneously occupied. This field is only
               needed if there is more than one cluster of disordered atoms
               showing independent local order.
               *** This data item would not in general be used in a
               macromolecular data block. ***