Creative Commons Licenses: Public Domain Tools

Creative Commons also has 2 public domain tools. 

CC0 enables creators to dedicate their works to the public domain. Some jurisdictions do not allow creators to dedicate their works to the public domain, so CC0 has other legal mechanisms included to help deal with this situation where it applies. This tool is often used by museums, libraries or archives. This doesn’t mean they are claiming copyright over those works, but rather they are waiving all possible rights they might have in other jurisdictions to the reproductions of those works.

The Public Domain Mark is a label used to mark works known to be free of all copyright restrictions. Unlike CC0, the Public Domain Mark has no legal effect when applied to a work. It informs the public about the public domain status of a work and is often used by museums, libraries and archives working with very old works.

These public domain tools are not equivalent to the 6 CC licenses.

They allow institutions, like museums, to indicate that a work is owned by the public. 
They can also be used by individuals to waive their rights to a work and dedicate it to the public domain.