Automated email rules are a great way to have your online system reward, send offers & thank you messages to your loyal customer base who order from you online.
Email rules create one-time coupon codes that are associated with the main code that you create (so, kind of like sub-codes). This prevents the issue of a customer distributing the code to someone else because it can only be used the one time, but keeps the main coupon code active and distributes another one-time code when those criterias are met again.