RoboHelp 2015 is logically version 12. The next version forward from RoboHelp 11. The User Interface (UI) has changed in 2015 to adopt the Ribbon as Microsoft Products did years ago. There are new capabilities in the Responsive output. For example, the ability to configure and employ "Filters" to hide some content in order to emphasize other content.
Of course there are many other features, but I believe those are the main selling points.