Visual Focus
Many accessibility aids need to identify the "focus point" where the user is working. For example, a blind-access utility must describe the text or object that the user is working on, and a screen-magnification utility pans out to enlarge whatever is at a particular point on the screen. Other utilities may move pop-up windows, so they do not cover "where the action is."
Sometimes, it is easy for an accessibility aid to determine this location; the operating system provides it when it moves the focus between windows, menus, or standard controls. It is more difficult to determine the location when an application uses its own method of indicating the visual focus within its window, such as highlighting a cell in a spreadsheet, an icon, or a windowless custom control. In these cases, to be accessible, the application must make its focus location available to other programs in the system; the convention for doing this is to move the system caret.