Reserved prefixes for local dictionary extensionsThe CIF standard permits the free incorporation of local and standard data names in any file. However, to work properly as an information interchange standard, CIF provides a facility to register certain prefixes which will guard against name clashes between arbitrarily chosen local data names.
Use of a reserved prefixA reserved prefix as registered in the IUCr list above may be used to generate unique local data names in the applications managed by the registrant.
DDL1 applicationsFor use in CIF data files based on DDL1 dictionaries, the reserved prefix may form the first element of any data name.
_foo_chemical_formula 'H6OC2' loop_ _geom_bond_atom_site_label_1 _geom_bond_atom_site_label_2 _geom_bond_distance _foo_geom_bond_comment C1 C2 1.528(4) normal C1 C3 1.358(3) short
DDL2 applicationsFor use in CIF data files based on DDL2 dictionaries, where the data name explicitly includes the category to which it belongs in the standard dictionaries, the reserved prefix must be used either as the first element following the point character separator between category and member components, or as the first component of the complete dataname, in which latter case it implies the formation of a local category. Only data names from the same category (standard or local) may be looped together in the same list construct.
_foo_chemical.formula 'H6OC2' loop_ _geom_bond.atom_site_label_1 _geom_bond.atom_site_label_2 _geom_bond.distance _geom_bond.foo_comment C1 C2 1.528(4) normal C1 C3 1.358(3) short loop_ _foo_symmetry_Wyckoff_group.notation _foo_symmetry_Wyckoff_group.count _foo_symmetry_Wyckoff_group.point a 2 -1 b 2 -1 c 2 -1 d 2 -1 e 4 1