DDL (Description Definition Language) is part of the MPEG-7 standard.
It gives an important set of tools for the users to create their own Description Schemes (DSs) and Descriptors (Ds).
The main agreement was that DDL had to use the XML syntax, support object-oriented semantics, as well as being able to validate structural, relational and data typing constraints.
Moreover, the strategy was to keep tracking and influencing the W3C community, specially the XML Schema, XLink, XPath and XPointer working groups.
At the 51st MPEG meeting, the adoption of the XML Schema syntax with specific MPEG-7 extensions was decided.