The idea is that when there is an activity of some sort e.g., annotation, a notification (in RDF) can be sent to a subject's inbox (that can be any thing
, a profile, a section, paragraph, ... essentially a URI). Senders and consumers can utilise that information for different purposes. https://rawgit.com/csarven/ldn/master/index.html#introduction
shows an annotation (as an aside) that was retrieved by first looking at the document's inbox, and then finding its notifications, looking into the notification to discover the annotation, getting the annotation and injecting it back into the document (as an aside).