A Rights Expression Language or REL is a machine-processable language used to express intellectual property rights (such as copyright) and other terms and conditions for use over content. RELs can be used as standalone expressions (i.e. metadata usable for search, compatibility tracking) or within a DRM system. RELs are expressible in a machine-language (such as XML, RDF, RDF Schema, and JSON). Although RELs may be processed directly, they can also be encountered when embedded as metadata within other documents, such as eBooks, image, audio or video files.
Notable RELs Notable RELs include:
ccREL An RDF Schema used by the Creative Commons project to express their licences. This same vocabulary has also been adopted by the GNU Project to express their General Public License (GPL) in machine-readable form. W3C Open Digital Rights Language ODRL The W3C Permissions and Obligations Expression (POE) Working Group has developed the ODRL recommendations for expressing permissions and obligations statements for digital content. The W3C ODRL Information Model offers a framework for the underlying concepts, entities, and relationships that form the foundational basis for the semantics of ODRL expressions. The aim of the ODRL Information Model is to support flexible Policy expressions by allowing the author to include as much, or as little, expressive detail about the terms and conditions for Asset usage, the Parties involved, and obligations. The W3C ODRL Vocabulary & Expression describes the potential terms used in ODRL Policy expressions and how to serialise them. The terms form part of the ODRL Ontology and formalise the semantics. The wide set of terms in the vocabulary provides the support for communities to use ODRL as the primary language to express common use cases. XrML XrML began with work at Xerox in the 1990s. After passing through several versions and separate projects, it later formed the basis of the REL for MPEG-21. MPEG-21 Part 5 of this MPEG standard includes a REL. METSRights METSRights is an extension schema to the METS packaging metadata standard. RightsML IPTC's Rights Expression Language for news media based on ODRL.
Use of a REL The function of a REL is to define licences, and to describe these licences in terms of the permissions or restrictions they imply for how the related content may then be used. "Licence" here may mean either:
A "well-known licence", such as GFDL, Apache License or a Creative Commons licence like CC BY-SA 4.0 etc. A pre-defined licence that is like these, but not so well known. Examples would be proprietary "shrinkwrap" licences. A specific licence that is created with individual terms and conditions, for content licensed from one party to another.
Well-known licences Use of a well-known licence is often chosen for its unambiguous simplicity: GFDL means the same no matter who is using it. Using existing licences also avoids the problems of licence proliferation. It is also practical to use such a licence, and to check that a project is complying with it, without understanding too much about what detail it entails. Merely knowing that "GFDL is acceptable to this project" and "All resources within this project use GFDL" is sufficient. In that sense, well-known licences are a way to avoid needing to use a REL to model the details of a licence, its name alone is enough. Despite this, a REL may still be useful with these licences. It provides a machine-processable way to identify the licence in use, avoiding naming issues and potential ambiguities between "Apache License" or "Apache 2.0 Licence". The authors of these licences also require a means to describe their internal details. Some software bill of materials (SBOM) products, such as Software Package Data Exchange (SPDX) do not use a REL but instead limit potential licences to a set of well-known licences, expressed through their local controlled vocabulary of SPDX ID. Each license is identified by a full name, such as "Mozilla Public License 2.0" and a short identifier, here "MPL-2.0". Licenses can be combined by simple Boolean operators AND, OR, and grouping ( ... ). However this still requires human intervention to check these licences for acceptability and for their effects when combined: the non-REL SBOM product cannot do this itself.
Pre-defined licence These are similar to the well-known licences, in that they're defined before their use and can be applied to many instances of licensing. Their difference is that as they're not well known, it's also necessary to explain what each of them entails, as the user is always likely to be encountering each of them for the first time. A REL provides the means to do this. Using licensed content within a project now requires evaluation of the statement, "Are there any resources within this project whose licence forbids a condition that the project requires, or requires a condition that the project cannot permit?". These might include a necessary ability to distribute copies of the project afterwards, or a condition for accreditation on a splash screen that might be unacceptable to some projects. In open source software development, it's also common for projects to create their own licence under their own project name, but for the details of this licence to be a boilerplate copy from a well-known licence, or even a reference to this licence. A REL should support this, providing a means for licences to be defined by sub-classing existing licences and possibly changing their behaviour. Many of these licences are little more than vanity licences, although other dependent projects must still be able to work with them.
Specific licences These are licences that are created as needed, for specific pieces of content, or specific end users. This is usually so that they may have use-specific conditions attached to them, such as expiry dates. Although these licences might be based on a standard boilerplate, each one is thus unique. Referring to them by name could not work as there's no single, stable name. It's thus necessary to use a REL to express each one in terms of its individual properties. Examples might include a time-limited contract to watch TV sport for a month, as paid for by an ongoing contract, and to watch this within the home but not to show it within a public bar.
Structure of a REL A REL may conveniently use an Entity–attribute–value model, as for RDF, to structure its description of a rights model. Such a model expresses itself as lists of:
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