ccREL

ccREL is the Creative Commons Rights Expression Language. It is the standard recommended by Creative Commons for "machine-readable expressions of copyright licensing terms and related information". See http://wiki.creativecommons.org/images/d/d6/Ccrel-1.0.pdf for the entire white paper about ccREL.

The ccREL is based on the RDF data model and allows web page publishers to use whatever particular syntaxes match their needs. They do provide "default" syntax schemes in RDFa and XMP.

Creative Commons have defined six pre-packaged licenses that seek to allow creators to retain copyright but to grant persmissions to make use of their work in specific ways. (There is also a CC0 license that grants all rights, placing the work in the Public Domain). These licenses all grant rights beyond "fair use" and are designed to maximize re-use of content. These pre-packaged licenses have been fairly widely implemented and tools have been developed to search various content repositories that support CC-licensed content. (See, for example, Creative Common's own search tool).

ccREL is Insufficient

Although the pre-packaged CC licenses are quite widely used, the ccREL language is insufficient to meet the rights requirements of most b2b publishers. The permissions and restrictions that can be expressed using ccREL are governed by the Creative Commons themselves: they do not permit others to extended the vocabularies themselves. The existing vocabulary is quite limited. According to the ccREL 1.0 whitepaper, the things that can be permitted are
 

c:Reproduction - copying the work in various forms.
cc:Distribution - redistributing the work.
cc:DerivativeWorks - preparing derivatives of the work

The one possible prohibition is

cc:CommercialUse - using the Work for commercial purposes.

The above permissions and prohibitions are not sufficient to cover the needs of most publishers for expressing rights and restrictions on content. You can confirm this for yourself by trying out various sets of permissions and restrictions you would like to enforce on your content and using the Creative Commons Choose a License workflow.