From Adobe blog: New InDesign Features Streamline Digital Publishing Suite Workflow
With CS6, Adobe has added ‘alternate layout’ functionality to streamline production of digital editions for a number of different platforms. See screenshot below!
The assets in the document are shared across each layout which makes it easier to keep content up-to-date. Say, for example, you find a typo in the article text just before publication. With Alternate Layouts you can make the correction in the document’s primary layout and then use the links panel to push the change to all the other layouts in the file.