I just watched a couple of videos related to notification.
1. Add subscribers to a notification. v1.1 http://www.youtube.com/watch?v=ewSXea7AExw&list=PL5227D1AFF2A40A6B&index=26&feature=plpp_video
2. Set the number of required acknowledgments for a notification. v1.1 http://www.youtube.com/watch?v=Tz4VT0DqVgc&list=PL5227D1AFF2A40A6B&index=27&feature=plpp_video
Based on the video I think subscribers means people that can be notified when event raised. And the acknowlegement is the acknowledgement when people get email alert.
For me our app need to register/subcribe PI points/tags to notification and write ack back to PI System. see the requirement here
can PI notification achieve this?