The process of providing a unique identifier for an element or entity that changes over time. Versioned elements retain their original ID but their version number is incremented to reflect a difference in content. This allows a reference to persist through the ID while allowing for either the specified version or the most current version of the element to be obtained. What is versioned, maintained, and referenced in DDI Lifecycle is the metadata itself, rather than the XML which expresses that metadata. While this might seem like a minor distinction, it has major implications for how applications are developed.