When using GTK+, typically you should use Window.setRole instead of this low-level function.
The window manager and session manager use a window’s role to distinguish it from other kinds of window in the same application. When an application is restarted after being saved in a previous session, all windows with the same title and role are treated as interchangeable. So if you have two windows with the same title that should be distinguished for session management purposes, you should set the role on those windows. It doesn’t matter what string you use for the role, as long as you have a different role for each non-interchangeable kind of window.