Customer Prepayments is a set of features to correctly handle deposits on your orders. If you receive payments on orders to apply to future invoices, those are Prepayments.
If you turn on Prepayments in BigBusiness 15, you will have all the tools you need to handle the accounting, importing, auto-applying, and reconciling of customer deposits.
To enable Prepayments, fill in the Prepayment Category in the Customer Prefs tool. When Prepayments are enabled, on the Payment you can choose Prepayment