3D user interface is computer user interface that is 3D and is spatial in nature and depends upon 3D interface devices, including output devices and input devices.
3D UI can be prototyped using Unity.
3D UI can be built for different input devices, such as electromagnetic tracking pens and/or hand tracking.
3D UI is most effective when it is in the personal space.
It is possible to have 3D user interface on a smartphone AR app, where the interface is completely in world space, and not in screen space.