The Benefits and Limitations to "Only one use per email address"
When creating or editing a promo code you have the option to restrict the promo code to only be used once per email address.
When this option is turned on, the customer can only use the promo code for one purchase on that specific store. If they revisit the same store and try to use the promo code again with the same email address they will receive an error at checkout. This is beneficial if you want to limit the number of times the customer can use the promo code on a store.
The limitations of allowing the promo code to be used once per email are mainly on promo codes with a dollar amount value. If the customer checks out and doesn't take full advantage of the promo code's dollar amount, they cannot place a second order using the remaining value.
If "Only one use per email address" is turned off, the customer can reuse the promo code infinitely during the life of the store.
The promo code page will display which promo codes have "Only one use per email address" turned on or off at a glance.