Rights Expression Language

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 (ie 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 include:

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:

Use of a well-known licence is often chosen for its unambiguous simplicity: GFDL means the same no matter who is using it. Using an 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.

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.

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