DIDL only defines the structure of a complex object. Any additional descriptive or administrative metadata about a container, item, or component must be stored in a metadata wrapper, called a descriptor. The MPEG-21 Rights Expression Language (REL) in Part 5 and the Digital Item Identification Language (DII) in Part 3 of the standard can be used to capture some of this metadata. Additionally, a descriptor may contain any non-MPEG-21 XML structure to capture preservation metadata.