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. 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. 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:     The REL defines sets of members for each of these three groups, and the permitted relations between them. In the example above there may be concepts of "Licences", "permissions" and "redistributing copies". Also there may be the relations, "A Licence may express prohibitions", and separately "Permission may be given to redistribute copies". Statements may then be made using the REL (these would be outside of the REL itself) such as:     This defines a new abstract licence, one that permits re-distribution of copies. Works may then use this Licence by referring to it,   This web page is licensed under FooCo's Distribution Permitted Licence. Note that although this hypothetical "Distribution permitted" licence has been expressed using the Creative Commons REL, it is "not" a Creative Commons licence. It merely uses the concepts "License", "permission" and "Distribution". Although it's not one of the Creative Commons licences defined by that project, it does share exact commonality for these terms: "Distribution" has exactly the same meaning and legal definition between them.