A Frame DOESN'T concerne the display. A Frame is only a sort of controler that permit to catch some specific action. A frame is called a frame because it could be organized into a stack (the framehandler).
By convention it could be good to associate a frame to a Screen but it's NOT the only one way to use frames.
Else There's no other UI hierarchy than the Desktop is the root widget and can contains only Screens. Other widget are completly flexibles. You can refer to the Widget Reference the see some specific internal widget hirarchy.
