The Elements tab in the Visual Designer shows a listing of all widget elements in the current record format:
You can click any element in this list to select it on the screen in the Visual Designer, view the element's properties, manually change the order of the elements using the Up/Down arrow icons, etc. The Auto Arrange by Element Position button will also allow you to resort the list of elements by their position on the screen.
The order of elements in the Elements list controls the tab order of input elements on the screen for Rich Display Files. In order to change the tabbing order for input elements on your screen, the order of the elements in this list should be changed to reflect the desired tabbing order.