The options are: Silence, Next Album (or previous depending on your autoplay setting), Artist Radio (of the artist you are playing), Similar Artist Mix, Decade Radio (of the album you are playing), and Library Radio.
Next/Previous album is default but you can change it yourself as you come towards the end of an album (swipe on mobile or scroll on desktop).
Check out the Howto thread: [How To] Plexamp 4.6.1
