I was reading a report by Cathy Davidson and David Theo Goldberg, focusing on the potential for shared and interactive learning made possible by the Internet. That led to search for existing plugins to develop some of those ideas on a website. This is what I found:
The combination of both enable further user interaction with the content, to a much finer granularity than in regular post comments, as they both allow to pinpoint specific moments, or pieces of text to comment into.
Here’s the link to the MIT Press report that started all.