Store credit is a very useful asset to any store in much that a same way a gift card is, as it keeps your customers shopping for items in your store and not a different store.
If a customer has store credit associated with their loyalty account it will be displayed in a box underneath their account details during a transaction. In this example, Arya has $13.00 in store credit on her account.
Always check with a customer before using their store credit. Some customers prefer to save it for later.
To use it towards a purchase click on the "Use up to xxx in Store Credit" (Shift + F3) button. A window will pop up allowing you to specify the amount of store credit to be used. Typically the customer won't have a desire to split their store credit, but it never hurts to ask.
Once store credit is used it's displayed as an item similarly to how other forms of payment display once entered. Here, the item's price is greater than the amount of store credit than Arya has, and so the remaining total is displayed. The customer will have to pay the remaining total with some other form of tender.
If the item being purchased is less than the amount of store credit the customer has then the item's total will automatically be entered as the default amount of store credit to be used. This is adjustable should the customer wish to change it, but they will never need to use all of their store credit or more than an item's cost to purchase it.
After the amount of store credit to be used is confirmed it appears as an item as usual. The remainder of the store credit left on the customer's account is displayed where their original credit total was. In this case they used $4.19 of store credit to purchase a book and have $8.81 leftover for their next purchase.