While Microsoft has not clarified anything about it, but what looks like is going to be a way to roll out an update for parts of the user interface. Since its a pack, we should see more apps and features made available. Right now, the UI update and Core Update go hand in hand. However, now that the team is split, additional user interface changes may get rolled out with Windows Feature Experience Pack. It is listed as one Feature on Demand for Windows 10 and Windows Server along with IE 11, Notepad, Paint, PowerShell, Print Management Console, and more. It can be easily noticed in the Abut section of the computer, which upgraded to 2004.