One of the most important aspects of DDEX is that it provides a common language for all the terms that are used in any music related business transaction. This common language is documented in the "DDEX Data Dictionary".
Somewhat counter-intuitively, there are multiple DDEX Data Dictionaries available from the DDEX Knowledge Base. However these "different" dictionaries are not separate dictionaries at all; instead they are all snapshots of the same underlying collection of terms but taken at different times and in response to the creation of a new version of an existing standard or the creation of a new standard.
DDEX maintains one ontology that provides the semantics (i.e. meaning) and syntax (i.e. structure) of all terms and concepts used in all of the DDEX standards. Each of these standards, however, only uses a subset of the terms and concepts defined in the full DDEX ontology.
If a user wants to know something about a term used in the, say, ERN standard, it would make that user's life more complicated if (s)he has to look at terms that are only used in the, say, DSR or MLC standards. Therefore, DDEX provides an ERN-specific data dictionary that contains only those terms and composites used in the ERN standard. The same applies to all DDEX standards.
DDEX not only develops communication standards, DDEX also maintains them. Maintenance of a DDEX standard involves, in many cases, changing the XML Schema Definition (XSD) of a message, and this, in turn, means that the data dictionary will be updated. Some of these changes will become part of the next version of the maintained standard, while other changes are discarded again when testing has shown that they do not work as envisaged. Consequently, there are always elements in the data dictionary that are not yet, and may never become, part of a DDEX standard.
And this means that it would be misleading to publish these parts of the ontology as part of a DDEX Data Dictionary. Thus there DDEX does not publish a complete DDEX Data Dictionary.
ONTOLOGY VS DD