The main component of the UGUI system is the Rect Transform. All other GameObjects have the basic Transform Component, but the Rect Transform works solely for the UGUI setup.
The Rect (short for rectangular) Transform is used to control the size and positioning of the UI elements in your Scene. Even in 3D space, all the UI elements are two-dimensional, so by using rectangular values for position and size, we can get a good idea of their look and placement. Figure 14.4 shows the Rect Transform for a Canvas GameObject.
By default, all UGUI components have a Rect Transform ...