@nawanp @Suiseiseki @goo

They actually are copyright licenses, from what I understand.

If something is public domain, it doesn't belong to anyone, and everyone can use it without a license at all.

However, some jurisdictions don't have a concept of something being released willingly into the public domain by it's author. Everything is always under copyright law in those jurisdictions, until the copyright expires. This means that, even if the author published a declaration saying "I release this work into the public domain," that is not legally binding and the author can still later sue you for using the work if the author changes their mind.

In such jurisdictions, an author can release their work under a license like CC0 to give others a license to use it in the same ways they would be able to use it if it weren't under copyright / if it truly were in the public domain.

Such a license would, by the way, not be a copyleft licence, because public domain is not copyleft. A derivative work of something in the public domain is not automatically public domain. For example, recent Sherlock Holmes movies and TV series are not public domain, even though the character of Sherlock Holmes and other figures appearing in the original works are now in the public domain.

A license that claims to release a work into the public domain but actually enforces derivative works to be released under similar terms is using copyright law to enforce that restriction. It couldn't do so if it weren't a copyright license.

Even a license that doesn't add such a restriction, and truly allows everything that public domain allows, is still a copyright license, because it grants those rights under copyright law, even if the resulting situation is equivalent to if the work were truly in the public domain and didn't require a license. You basically *emulate* public domain with such a license.