Comment 5 for bug 388556

PID is the _NET_WM_PID window property, specified in:

 http://standards.freedesktop.org/wm-spec/wm-spec-1.3.html#id2507760

If the process creates a window at all, that is what AllTray will check for and dock. The window must have _NET_WM_PID set to be AllTray's child (or a descendant thereof that is in the same PGID).

If the window goes away entirely (is closed), the icon will disappear, as well, and AllTray will begin looking for the application to reappear if the application is AllTray's direct child (and thus AllTray can tell that it is still running).