The crossed-out circle means banning an item, so that it is not played automatically.
In the past, this could only be applied to items in the Roon library, i.e., clicking the hollow heart made it a filled heart (meaning „favorite“) and the next click made it the crossed-out circle (meaning „banned“).
This meant that it was impossible to ban items that were NOT in the library, meaning that they could still be included in Roon Radio and playlists.
To improve this, Roon made it possible to ban all items, whether they are in the library or not. And therefore, the option had to be moved to a different place. This happened with an update in August:
See the updated documentation:
If you want to mark albums for later listening (or other purposes) it’s probably a better idea to use Roon tags: