This post is someone related to the part v in the series, subject lines.
The subject of the email should reflect the topic of the body. When replying to an email thread, you should only reply with content relevant to the main topic. Do not ask an unrelated question, or start writing about what you had for lunch.
If you have a new topic, compose a new email!
It may not be entirely obvious why this sucks, I’m going to go into a little more detail than usual. This sucks because it makes it impossible to reference the email in the future. When I do a search for something, I usually ignore subjects that don’t pertain to my search. If I give the thread a label based on the content, when I reference the label in the future I will probably think it has been incorrectly labeled. It also makes it more difficult to resolve the email thread. It is also more likely to spin out of control, especially if there are a lot of CC’er in on the thread (see: PS@E IV).
Lay off that reply-all button.