It would need to be "some other license". There are countries where you can't just hand stuff over as public domain. In these countries the author would retain all right.
I've heard this a lot, but is it true? djb's page at https://cr.yp.to/publicdomain.html makes an argument that copyright can be waived anywhere in the world.
djb is wrong. Although it probably doesn't matter much, practically speaking.
There is exactly one way how a work can become "gemeinfrei" (public domain): expiring copyright.
So no, you can't place something into the public domain.
(There is a minority opinion, as always, among legal scholars, but the big majority says "no way").
You might be able to argue that your wish to place something into the public domain can be re-interpreted as relinquishing the "Verwertungsrechte" (rights of exploitation).
That's what djb seems to argue, and it is a sensible stance. i suppose many courts would agree.
But it's not as clear-cut as djb claims. And who needs ambiguities there?
To add on top of that: In germany it's even impossible to sell the full copyright (Urheberrechte). You can waive some (the right to be named as the author) and you can sell the rights to use the work (Verwertungsrechte) but some others can not be transferred.
For example the right, that the work may not be used to defame the author and may not be used in such contexts. The term copyright translates very badly to German law. (Sorry, on a mobile connection, so just one short example)
It would need to be "some other license". There are countries where you can't just hand stuff over as public domain. In these countries the author would retain all right.