The concept of abstract types is indicated by xsi:type and is used in MPEG-7 and MPEG-21 respectively.
Interestingly, this information is indeed comparable with the mobility characteristics as defined in MPEG-21 DIA UED.
Lambert continues, "Another significant obstacle to the adoption of XrML (and ISO MPEG-21
REL) is the claim by ContentGuard that XrML in particular, and any REL in general, are covered by ContentGuard patents.
This paper proposes a hierarchical learning design framework based on the MPEG-21 standard.
To achieve these objectives, the SLDF will be implemented within the broader based MPEG-21 multimedia framework.
The MPEG-7 and MPEG-21
standards have been developed to support such requirements.
Without doubt, the most complete initiative trying to identify and represent the context for generic multimedia applications has been carried out by the MPEG community by means of the 7th part of its MPEG-21
The adoption of MPEG-7  for describing metadata related to the digital items and of MPEG-21
 for describing metadata related to a governed content (i.e., with an associated license), as proposed in this paper, is a common approach used by an increasing number of scientific communities.
Other standards of interest to the publisher and library communities are NISO Z39.87, which uses metadata to describe images; MPEG-21
, an extension of earlier versions of the MPEG standard that uses XML to describe content elements; and PDF/A (PDF/Archivable), a version of Adobe's widely used PDF that should remain stable over time and therefore can be used for digital preservation purposes.
Rosenblatt acknowledges, I pointed out in the article that RealNetworks withdrew sup port from XMCL during the MPEG-21
competition for Rights Expression Languages in December 2001.
As MPEG-7 and MPEG-21
are the dominant standards for multimedia application support, the research in this area is mainly based on them.
Coverage includes the fundamentals of digital video compression, digital video coding standards, transcoding algorithms and systems architecture, optimization of transcoding performance, video transport transcoding, system clock recovering and time stamping, transcoder video buffer and hypothetical reference decoder, cryptography and conditional access, application and implementation of transcoding, universal multimedia access with emerging standard MPEG-21
, and the end-to-end test bed.