Credit can be used to make purchases and bid auctions. It cannot be used to pay for international shipping charges or shipping insurance .
For example, if you have 100,000 yen of credit and 30,000 yen of funds, like the example in the image below, you can purchase items or bid on Yahoo! Auctions up to 130,000 yen.
Credit will be used automatically once you have exhausted your funds balance and will be treated as if you had a negative balance in your account.