1. Delimiter
Based on delimiter configured under Admin >> Mail Server Settings >> Delimiter
If the email subject contains a delimiter matching a request ID, then it appends the email as a conversation to the ticket ID present.
2. Message ID
Let's say, a request is raised by User A along with User B in the CC (Request ID: 123). Now, if the User B decides to create a request of their own, but instead of composing a new email if they Reply All to User A's email, clear the subject and User A's email address, and send the request, still it will be appended to the Request 123. This is due to the Message ID present in the email header.
Even though the delimiter is not present, the application recognizes the Message ID with the request.
The Message-ID will be present in the header.
Let me show you the example in real-time using a thunderbird email client.
Let's take
From the sent folder of Req1
From the Inbox of Req2
Now, I reply all from the same thread, remove req1 and add a new subject.
I will use, Edit as a New message to create a new ticket
From Sent folder