It will probably work just like the current Kindles. It has internal memory. Any ebooks that are in the cloud are considered part of your archive. You select ebooks from your archive and they are downloaded to your device. One there, they stay on the device until you remove them.
Most ebooks that you get from the store are usually sent to the device immediately (you can choose which device if you have more than one). Once you remove a book from your device, they are in the archives, in case you want to read it again, or load it onto a different device.
You can have multiple devices on the same account (I have my Kindle, Kindle for PC, and Kindle for Android on my phone and tablet). You can load the same book onto multiple devices and have it sync reading locations. That way, when I stop reading in my Kindle and decide to read while I'm out (without my Kindle), the phone app will start off where I left off in the Kindle.