Within Nabble: when you take a threaded view of some topics, you may see that people have replied to different points within the topic. Compare these screen shots:
— a single trail of each person replying to the previous message (Grant replied to John's message, Delphine replied to Grant's messages, and so on)
— JonJon replied to the opening message, then Grant replied to the opening message, then each thread grew.
See also <
http://en.wikipedia.org/wiki/Threaded_discussion>.
Regards
Graham