The generic function interface-extend-title is called by the system with an interface and its title before actually displaying the title on the screen. The result must be a string, which is actually displayed. There is no requirement for any relation between the title argument and the result.
The return value string is the title to display on the screen.
The default method uses the values set by set-default-interface-prefix-suffix. You can specialize interface-extend-title to get other effects.
interface-display-title
set-default-interface-prefix-suffix
Window titles
Controlling the appearance of the top level window
CAPI User Guide and Reference Manual (Unix version) - 3 Aug 2017