Notes, or sub-comments may be added to most items on a workpaper. This is done by pressing the comment (speech bubble) icon visible when you hover over the work item.
This opens a comment box as follows:
These notes may be cross referenced as per normal comments. Each reply will appear under the item it is responding to.
Notes may be added to normal comments. The font is slightly smaller than a normal comment. This is demonstrated in the screenshot below. (A normal comment may be added instead of a note but this will not be appended to the work item.)