The Internal Name/ Coupon Name is an identifier you can use within your company to remember which coupon campaign is which. This name will never be shown to customers, unless you choose to include the field through the API or email parameters. Coupon codes, are the code that your customers will enter to redeem the coupon these can only contain lowercase alphanumeric characters, ‘_’,’+’,and ‘-‘.
When entering coupons within the UI regardless of the case they are entered in the system will save the coupon code in lowercase. Despite this customers should be able to enter the coupon code as lowercase or uppercase when checking out and have the coupon applied to an applicable purchase.
We have some additional information on creating coupons in our docs here: https://docs.recurly.com/docs/coupons#section-create-a-coupon