Soapbox has a "tiered" model of user login, as opposed to offering "groups" with explicitly assigned access to content.
There are two levels of user - Registered and Special. "Registered" users can access any content you set the "Registered" level.
Likewise, "Special" lever users (Authors and Editors) would be able to access any content you set at the "Special" level.
However, Special level users would also be able to access any content set to "Registered."
So you could have some content that the Registered and Special users could view, and other content that only the Special users could view.
But you would not be able to have some content that only the Registered users could view, and other that only the Special users could view.