Answer:
CC licenses are operative only when applied to material in which a copyright exists, and even then only when a particular use would otherwise not be permitted by copyright. Note that the latest version of CC licenses also applies to rights similar to copyright, such as neighboring rights and sui generis database rights. Learn more about the scope of the licenses.
This means that CC license terms and conditions are not triggered by uses permitted under any applicable exceptions and limitations to copyright, nor do license terms and conditions apply to elements of a licensed work that are in the public domain.
This also means that CC licenses do not contractually impose restrictions on uses of a work where there is no underlying copyright. This feature (and others) distinguish CC licenses from some other open licenses like the ODbL and ODC-BY, both of which are intended to impose contractual conditions and restrictions on the reuse of databases in jurisdictions where there is no underlying copyright or sui generis database right.
All CC licenses are non-exclusive: creators and owners can enter into additional, different licensing arrangements for the same material at any time (often referred to as “dual-licensing” or “multi-licensing”). However, CC licenses are not revocable once granted unless there has been a breach, and even then the license is terminated only for the breaching licensee.
Please note that CC0 is not a license; it is a public domain dedication. When CC0 is applied to a work, copyright no longer applies to the work in most jurisdictions around the world. Therefore, references to dual licensing arrangements like the one above are inapplicable to CC0.