Cocoa Programming for OS X: The Big Nerd Ranch Guide

What Notifications Are Not

A notification center allows objects in an application to send notifications to other objects in that same application. When programmers first hear about the notification center, they sometimes think that it is a form of interprocess communication – that they can create an observer in one application and post notifications from an object in another.

Notifications do not travel between applications, at least not via NSNotificationCenter. (Look into NSDistributedNotificationCenter if you need to pass notifications between applications.)