This threading mode means that if messages do not reference a previous email in a thread with the same message-id

How do email chains work?

An email thread is an email message that includes a running list of all the succeeding replies starting with the original email. The replies are arranged visually near the original message, usually in chronological order from the first reply to the most recent.

How do email threads work in Gmail?

When people reply to an email, Gmail groups responses together in conversations with the newest email on the bottom. A conversation breaks off into a new conversation if the subject line changes or the conversation gets to more than 100 emails.