When you redeem an ad credit (mon-monetary gift), it’ll pay for all advertising costs that you accrue on your account. This includes every ad running on your ad account, in addition to the one you created to redeem your ad credit. As long as you’re running an ad, your ad credit will be charged daily until it runs out or expires.
If you have WAC, your campaign will draw from this balance first. If your ad is still ongoing when the WAC is used up, it will draw from your FWC (primary payment method). Note: This feature might not be available to you at the moment.