Oracle has bundled Order header workflow with header level invoicing , this is just like other header workflow , but invoicing activity is added to order header. In other type of flow Invoicing activity is at lline workflow.So when ever line is fulfilled , it progress to Invoicing activity and create invoice in AR.So if an Order has 10 lines , and they reach to Invoicing activity at different time interval , then system may create 10 invoices .But if we choose Order Header workflow with Header Level Invoicing.when ever line is fulfilled it will go to the activity Fulfill - Continue Header Flow and inform the Order header that it is ready for Invoicing, Once all the lines inform that they are done with the fulfillment , Order Header will progress and complete the Invoicing activity . In this case there will be only 1 invoice for all the lines in the Order.
Please Note that here we are talikng
1. Order header work flow - Order Flow - Generic with Header Level Invoice Interface
and to support that we have to make sure that we choose Line level work flow as - Line Flow - Generic with Header Level Invoice Interface
NOTE - we came use fulfillment Set too if we want to invoice all the lines together by adding all the lines in same fulfillment set.