When you first open a new billing plan, several fields in the Customer Information, Billing Options, and Billing Default Overrides group boxes are pre-populated. These values come from setup that you define at the contract level on the Billing Options page or in the Billing Plan Detail template.
If no values were specified on those pages and you leave these fields blank on this page, the system uses the values for the billing business unit, bill type, and bill source that you defined for the Contracts business unit assigned to this contract, on the Contracts Definition - BU Definition page.