And now I am replyting to the thread as a whole by using the blue “Reply” button next to the “Share,” “Bookmark,” and “Flag” buttons. The system treats these ways of replying differently.
The difference is that when you click the “Reply” button under someone’s comment, that person is notified - as you just were when I made this comment. It’s also possible to view the comment someone was replying to, even if it’s much earlier in the topic. If you use the blue “Reply” button at the bottom, you reply to the topic as a whole, and only people who are “watching” the topic will be notified.