Under the recurring section on a product have a "Apply coupon to recurring events" box that can be checked.
When a customer makes a recurring purchase and has applied a coupon, the coupon only deducts from the original payment, and not the recurring payments. Having the mentioned box checked will apply the coupon to the future recurring payments. Some of us use the recurring payment as a way to provide flex pay to our customers. If a coupon is applied, we have to go in and change the recurring amount on their order to reflect the coupon discount. This takes time and can also be missed, causing customer complaints.