Reply emails are used to follow up on previously sent emails to create an email chain commonly seen when a person replies to your message.
It's essentially like going into your Sent folder and replying to a message you've already sent.
Reply emails will have the normal Re: shown at the beginning of the subject line on a replies.
To change an email from a New Thread (new email) to a Reply, change the Type when composing the email.
Composing the First Email in Outreach
Composing a Reply Email in Outreach