Introduction
UI Billboard is simply a container that registers itself with the UI Billboard Manager to rotate its transform to face the Camera.
For one single UI element, you could assign this component to that element itself.
If you have multiple UI elements next to each other – for example you might have a StatBar and an InfoIcon, its best to create a parent with this component on, and then add the StatBar and InfoIcon as children. That way, when rotation happens, it all gets rotated together, and stays in line.
Setting up
There is a prefab UI Billboard located in /prefabs/UI.
Setting up without the prefab is as simple as adding the component.