AR is accounts receivable. The customers owe you money. This is for B2B (business to business) customers with a good relationship. You can use AR to complete (invoice out) transaction without receiving payment and collect the payment later. You can also charge interest on AR.
https://www.investopedia.com/terms/a/accountsreceivable.asp
Store credit (accounts payable) is opposite, you owe customer money. Store credit is used for sales return without giving customer money back.