Basic anthentication, you can configure from the account setting.
When you’re not ready to show the world your masterpiece, use basic authentication, IP address or page viewing limit to view unpublished work.
When you have a selected content in your page where you only wanted members from a certain group to have access to the content.
Eg. Content specially for members.
The restriction is set in the article editing screen, where the page is still accessable. However, a certain content in the page itself is not available when they are not logged in.
Eg. premium paid member and basic free membership.
This restriction can also for a group of articles. This will be set at the group editing screen.