Wikimedia Commons explains the concept of multi-licensing as:
Multi-licensing means releasing content under multiple licenses. Doing so gives more freedom to users of the content, as they can choose which of the licenses best suits their needs. Multi-licensing with restrictive licenses may be desirable for compatibility with the licensing scheme of other projects; also, multi-licensing allows people who create derivative work to release that work under a restrictive license only, if they wish — that is, it gives creators of derivative works more freedom with regards to which license they may use for their work.
Scholarpedia's default content license does not allow commercial use, which makes our license incompatible with Wikipedia. However, Scholarpedia authors retain the right to license their contributions with additional licenses. This can be useful if you want to donate your contributions to Wikipedia or other projects. We have made the following template that can be included in articles to release them under the CC-BY-SA 3.0 license:
Any contribution made to an article that contains such a license note is also released under this license. Contributions can be made without additional licenses by removing such notices, however the article's curator may not accept such contributions. Note, however, that while authors may cease offering their work under Creative Commons licenses, older licenses are not revoked. For example, if the content released under CC-BY-SA 3.0 is incorporated into Wikipedia, the authors cannot revoke this license and demand that Wikipedia removes the content. However, the authors can stop offering their article with the CC-BY-SA 3.0 license.