What is the difference between Indicator Plugin and Notification Area?

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http://askubuntu.com – I don't really understand why Ubuntu has two "notification area" style things that seem to serve the same functionality. I can't use only one because some of the programs seem to only work with Notification Area (e.g. Orage) and some only work with Indicator Plugin (e.g. indicator-sound). I noticed that the network manager applet will prefer to insert itself into Indicator Plugin and if that isn't available, try Notification Area. I don't understand why we have/need both? Is it normal to keep both running? (HowTos)