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Rights Expression Language (REL) is a machine readable language used in digital rights management (DRM). REL supports consistency and reliability among different systems and services and guarantees end-to-end interoperability, which allows different systems to work together.
REL's main function is defining licenses, meaning to define permission and restrictions with regard to document content usage.
REL features the flexibility to support multiple business models, richness and extensibility for DRM. It facilitates the identification and association of digital rights to digital content. Most RELs are expressed in XML, but any format may be used, including simple text.
RELs are usually embedded as metadata (data, such as creation date, language used and tools used to create media) in documents like MP3 audio, downloaded video or e-books.
Examples of notable RELs include Open Digital Rights Language (ODRL), Creative Commons REL (CC REL), and Moving Picture Experts Group (MPEG-21). A common online REL is the General Free Documentation License (GDFL), which gives users free permission to copy and distribute a work that is often subject to copyleft - the opposite of copyright.