You now have the option to reset the statistics for any Transactional Group to all zeroes. This won't delete any of the data behind the statistics, but will just start counting all unsubscribes, bounces, opens, clicks, and complaints from zero again. So, you can still see all of the email address that unsubscribed, bounced, opened, clicked, and complained historically.
Why would you want to reset your statistics?
There are many reasons, but the most common one is if you're sending test emails as part of a Transactional Group, and you're ready to stop sending and start sending for real. You may not want the counts from the test emails counted in your official Transactional Group statistics.