UI 2.1.1 is a package which contains extra nodes for H3DAPI. The source code for the package is released. UI is binary distributed with H3DAPI 2.1.1 and H3DViewer 2.1.1 packages for Windows.
The UI package is a collection of haptic user interface nodes that allows the user to create haptic buttons, slider bars etc. This can be useful when creating an haptic application in which the user interface should respond to haptic interaction as well as mouse interaction.
Grab UI from the downloads section.
For more information see the entry about UI in the wiki: