Along with numerous overall user interface changes landed in Quantal, Unity greeter has received minor yet numerous refinements, transforming the greeter into a more polished, visually precise and worked-on-detail login experience.
The latest updates bring a new version of Unity greeter in Quantal Quetzal, further adding subtle user interface changes.
At login, in order to select a different session, the user clicks the login box's Ubuntu logo, action that exposes the available sessions on the box, clicking on a session (such as GNOME), opens the clicked session.