prism, interaction request: why is binding set to Notification.Content?

I am reading chapter 6, Advanced MVVM Scenarios of the Prism 4.0 documentation.  On page 77, the sample XAML binds theTextBlock to the Notification.Content property:

      <TextBlock TextWrapping="Wrap" Grid.Row="0" Text="{Binding}"/>

Why/how does this XAML specify that the binding is to the Notification.Content property and not theNotification.Title property?