I wasn't able to find one. The closest triggers I could find in the DT Register email configuration are for
- [STATUS] - Status (Active, Pending, Cancelled, Waiting)
- [PAID_STATUS] - Paid Status (Paid, Not Paid)
I can certainly see the value of generating an automatic thank you email, for example, after someone attends using that status as the trigger.
Maybe something to add to the wish-list for a future update.