The service credit balance is the amount of credit the subscription has in Service Credits. Adding or deducting service credits can be done in the Subscription Actions menu. This screen will show the history of previous service credits as well as give the ability to give or deduct service credits.
Please be aware that this article is related to Advanced Billing's Subscription Summary function. The subscription summary is the place to perform all tasks necessary to manage customers’ purchases through Advanced Billing. You can learn more about the Subscription Summary by checking out this help article: Subscription Summary Overview.