Prepayments allow for external payments to be added to a subscription. These do not count towards a specific invoice, instead, they applies to the subscription as a whole. This tab allow the addition of prepayments by choosing a payment method from a variety of external payment methods, as well as spots to include details, a memo, and an amount.
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.