February 9th, Microsoft released some more details of their Windows 8 ARM edition, and while doing so also publicized some concepts for the new Microsoft Office package.
The new design for the Office applications includes an overall flatter interface, inspired by Microsoft’s Metro theming. You can also see there’s a bit more work space, some of which subsists due to the fact that ribbon content seems to be hidden.
Also, Office 15 was developed with tablet PC’s in mind, as it was designed to work well with touch screens and works without consuming too much power.
While watching the video about Windows 8 ARM edition, I noticed there was a sign-in button near the top right hand corner of each Office application. 
Because there weren’t too many details about Office 15 in this short little video, it’s hard to say what it’s for. My guess is that it might be an easier way of saving and opening Office documents from Skydrive, but that’s just speculation and I could be wrong. We will only know in time.