Introduction
Formerly, UI implementation was done in UDK using UIScenes , a part of the Canvas tool. These were internally integrated in the editor but in some ways limited (although it is possibly a shame UIScenes were deprecated completely). Scaleform Corporation, just before it was acquired by Autodesk, paired up with Epic Games to allow Flash based .SWF content to be imported into UDK. This allows designers access to the timeline-based, vector-based graphic animation Flash permits, and also access to ActionScript commands that drive cursors, button rollovers, clicking actions, and user input. Additionally, adding the Scaleform GFx Media Player extension within Flash provides a way for designers to preview how the UI content will look in game directly as they work, since they don't have to export, compile and build their game, and so forth.
The overall design process incorporating Scaleform for UI is to come up with a plan for the UI features, then do the art (or at least placeholder art...