How does case merge and email threading work in Service Cloud?

Case Merge allows agents to combine up to 3 duplicate cases into a single master case. Activities, emails, and comments from merged cases are transferred to the master. Merged cases are closed and marked as duplicates. Email Threading keeps all email replies on a case connected by embedding a thread ID (e.g., ref:_00D..._500...:ref) in email footers. When a customer replies, Salesforce matches the thread ID to the original case and appends the reply as a new email activity rather than creating a duplicate case. Email threading tokens are configured in Email-to-Case settings under Support Settings.