One of the most important aspects of DDEX is that it provides a common language for all music related business transactions. This common language is documented in the "DDEX Data Dictionary".
Rather surprisingly, there are multiple DDEX Data Dictionaries available from the DDEX Knowledge Base and, although this is counter-intuitive, there is a good reason for this. The first thing to note is, though, that these "different" dictionaries are all snapshots of the same underlying collection of terms; they are not distinct dictionaries.
DDEX maintains, behind the scenes, 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.
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 standard. Therefore, DDEX provides a data dictionary specifically for the ERN standard. And the same applies to all DDEX standards.
Critically, though, each of these standard-specific dictionaries are generated from the DDEX ontology that contains all standards maintained by DDEX and that includes published standards as well as those under development.