The UI uses a coordinate system relative to the camera it is assigned to. All of the UI layout is through the Transform component, which is interpreted differently for UI elements.
The coordinates go from
(-1,-1) (bottom left) to
(1,1) (top right). This
results in the center of the camera being marked by position
(0,0). To put a
UI element a quarter along the screen, as shown here:
This UI element would be at position
new Transform(Vector.New(0.5, 0));
The scale of UI elements is relative to the camera's viewport - meaning that a
(1,1) would result in a UI element that takes up the entire screen,
while a scale of
(0.5,0.5) would be half the size of the screen vertically and
horizontally. Keep in mind that this UI scale system is affected by the aspect
ratio of the camera, and as such the aspect ratio should be accounted for when
specifying scale dimensions, otherwise the scale may be distorted or stretched
in an unwanted way.
The rotation works exactly the same way as a normal world object, through the Transform component. The orientation is determined in radians.
When adding a UI component, you must specify the camera that the UI will be rendered on. The UI element will exclusively be rendered on this camera, and all calculations of scale and positions will be relative to it.