Customer Payment Terms

Does anyone have documentation on the how the customer payment terms hierarchy works in PeopleSoft 9.1 for Order Management / Billing?  We currently have terms set up for each bill to customer, which clearly overrides what is set up at the business unit level.  There is a need to override the bill to customer's terms for certain order groups on the sales order.  We set the new default terms code up on the order group, but that didn't seem to override what was set up for the bill to customer.  Is there a way to override the payment terms that