A credit coupon is for cases where you would like to give a client a certain amount of credit towards their order. Many photographers include X number of dollars worth of prints with their sessions and the credit type of coupon allows you to do this.

Credits are setup so that they do not have to be used on a single order.  Let's say you give a credit coupon to a client with a value of $100.  They place an order for $75.  Their credit still has $25 on it.  They will be able to use that $25 on subsequent orders.  Once the credit gets down to $0.00 it will no longer be usable.