CCPlus allows you to clearly add a human, legal and machine readable Creative Commons (CC) licence to your work PLUS clearly highlighting to users how they can obtain any additional rights not granted by the CC licence.
I must admit that I don’t think that I have seen the use of Creative Commons CCPlus (see press release December 2007) in the “wild” but on closer inspection I’m starting to wonder why.
Adopting CCPlus and applying to web based content is achieved through simple markup that can include the familiar CC button and a customised commercial licence button with the rights holder name on it (scalable vector graphic available here).
+ 
The CCPlus wiki page and technical guide has more information regarding adding CCPlus / CC+ to your site, some useful Use Cases are also included on the wiki page:
- Restrict commercial use with a CC license with the NonCommercial condition, and then use a separate agreement with some party (could be yourself or third-party) to broker commercial rights (licensing, sales, reproduction, etc).
- Require that adaptions are shared with a CC license with the ShareAlike condition, but offer a separate agreement (as above) for parties that do not want to release derivatives under the same license. Similar use cases for offering a private agreement for parties that wish to avoid fulfilling the Attribution or NoDerivatives properties of applicable CC licenses.
- Offer a private agreement for parties that require one (eg for institutional policy or insurance reasons), even if their use would be within the scope of the public license grant.
- To implement some type of Street Performer Protocol system to put works to the public domain or into another license, preferably more free and in the community interest.
By adopting and applying CC licences to your work you are in effect “dual licencing” and personally I think that this approach of adopting CCPlus helps overcome the perception that the content creator has given up all rights by applying a CC licence, eg for OER use where cc-by-nc is a typical applied licence the + licence would help clear any potential ambiguity.
For more information on Creative Commons the documentation page is a great place to start, for example if you are a content creator this page provides “best practice” for marking up text, audio, image and video resources as well as more specific file formats.