Section 2.2.7.4.3. Semantics of data items

(3) While the STAR File syntax allows the identification and extraction of tags and associated values, the interpretation of the data thus extracted is application-dependent. In CIF applications, formal catalogues of standard data names and their associated attributes are maintained as external reference files called data dictionaries. These dictionary files share the same structure and syntax rules as data CIFs.

(4) At the current revision, two conventions (known as dictionary definition languages or DDLs) are supported for detailing the meaning and associated attributes of data names. These are known as DDL1 (Hall & Cook, 1995) and DDL2 (Westbrook & Hall, 1995), and they differ in the amount of detail they carry about data types, the relationships between specific data items and the large-scale classification of data items.

(5) While it may be formally possible to define the semantics of the data items in a given data file in both DDL1 and DDL2 data dictionaries, in practice different dictionaries are constructed to define the data names appropriate for particular crystallographic applications, and each such dictionary is written in DDL1 or DDL2 formalism according to which appears better able to describe the data model employed. There is thus in practice a bifurcation of CIF into two dialects according to the DDL used in composing the relevant dictionary file. However, the use of aliases may permit applications tuned to one dialect to import data constructed according to the other.