RadTextBoxElement is created for the purpose of reusability, as it is not a control, but an element (* Refer to Telerik Presentation Framework section about the difference. *) e.g. RadTextBoxElement can be used in RadGridView control.
FillPrimitive and BorderPrimitive set the overall background and border properties of the control.
The most important node is RadTextBoxItem which hosts the standard.NET TextBox control.