The caption of a window will be displayed in its title bar; on the X Window System, the title bar is rendered by the window manager, so exactly how the caption appears to users may vary according to a user's exact configuration. The caption should help a user distinguish this window(RForm) from other windows they may have open. A good caption might include the application name and current document filename, for example.
Return value:
The caption of the window, or null if none has been set explicitly.