The mPeG-21 standard has been developed by the Moving Picture Experts Group (MPEG) Committee as an open framework for the delivery and exchange of multimedia objects. It must provide the flexibility required to describe complex audiovisual resources and support any media type and genre. The modular architecture of the MPEG-21 standard allows implementers to pick use case-specific parts of the 12-part standard without losing standard compliance. Part 2 of this standard is the digital Item declaration language (dIdl). DIDL uses five basic concepts for describing complex digital objects. The semantics of these concepts are more abstract than the sections in METS. Containers can group containers and/or items. An item can group further items or components. A component groups resources. All resources within the same component are regarded as semantically equivalent. DIDL defines a resource as an individual bytestream that contains the actual content of an item and can either be embedded into the DIDL description or referenced.