Windows 8 is available in three different editions, of which the lowest version, branded simply as Windows 8, and Windows 8 Pro, were sold at retail in most countries, and as pre-loaded software on new computers. Each edition of Windows 8 includes all of the capabilities and features of the edition below it, and add additional features oriented towards their market segments. For example, Pro added BitLocker, Hyper-V, the ability to join a domain, and the ability to install Windows Media Center as a paid add-on. Users of Windows 8 can purchase a "Pro Pack" license that upgrades their system to Windows 8 Pro through Add features to Windows. This license also includes Windows Media Center. Windows 8 Enterprise contains additional features aimed towards business environments, and is only available through volume licensing. A port of Windows 8 for ARM architecture, Windows RT, is marketed as an edition of Windows 8, but was only included as pre-loaded software on devices specifically developed for it.