Motif has perhaps one of the cleanest models. A highlight border is displayed around a component when it has focus. This is consistent for all types of components. Other windowing systems use mixed models. If a button has focus, a dashed border is drawn around the button text.
External navigation is concerned with moving the focus between components in an application.
This model makes use of the Tab key to navigate between components. Other keys, such as arrow keys, are used for internal navigation. The tab order determines the sequence of components that receive keyboard focus. The tab order commonly follows the layout of components from left to right and top to bottom. The left to right tab order was probably chosen because it reflects the reading direction of Western cultures.