There are two ways to comment in response to a post. You can jump directly to the ‘Submit Comment’ box and use it. That will result in a “primary” comment. The other way to comment is to respond to an existing comment by clicking ‘Reply’ just below the comment. That will result in a bullet-point, nested comment (the nests are enabled five levels deep). For example, see my comment here. Using the ‘Reply’ comments allows for sub-discussions and cuts down on some of the need to scroll up and down the comment papyrus.