Unlisted is an available category for users who want to share their documents publically. The difference with unlisted documents is that your documents will not be listed in the main base of public documents, even if they have been made public.

These documents become visible only to people who you share a link with via the available sharing options.
Sharing link can be found in Host tab.
They are not visible in search engines or on your Public Profile. If you want the documents to be made visible on your Public Profile, you have to assign them a category.