Add Features - This is the new name for "Anytime Upgrades" - you buy a new product license key, enter it, and after checking its validity, Microsoft automatically downloads and installs the new product to your system. In Windows 8 RP you can for instance upgrade to the MCE (Media Center Edition) by entering the (free) key:
if you have the Professional edition of Windows 8 RP installed already.
If you have the core edition of Windows 8 RP you need first upgrade that to Professional by entering the product key:
and let that upgrade perform before you can apply the MCE upgrade.