That was Jonathan's point.
If xfce4-notifyd provides the same functionality as notification-daemon to the API consuming application then it must replace&conflict the virtual package notification-daemon thus making xfce4-notifyd the present implementation of notification-daemon (of course for all sorts of reasons no conflict but runtime selection makes a bazillion times more sense).
That was Jonathan's point.
If xfce4-notifyd provides the same functionality as notification-daemon to the API consuming application then it must replace&conflict the virtual package notification-daemon thus making xfce4-notifyd the present implementation of notification-daemon (of course for all sorts of reasons no conflict but runtime selection makes a bazillion times more sense).