Project Infinite will enable users to seamlessly and securely access all their Dropbox files from the desktop, regardless of how much space they have available on their hard drives. Everything in the company’s Dropbox that you’re given access to, whether it’s stored locally or in the cloud, will show up in Dropbox on your desktop. If it’s synced locally, you’ll see the familiar green checkmark, while everything else will have a new cloud icon.

Dropbox announces Project Infinite, where all your dropbox files show up in the file system but you don’t have to have the whole files downloaded. It then fetches them on-demand. 

Currently if you’ve more files in your dropbox than storage space you have to use selective sync to prevent them from filling it up.

It’s exactly like BitTorrent Sync’s pro feature. Here it is in action to give you an idea how Project Infinite may work: