DP 91 is for resource related billing. In DP91 invoice will be generated to the cutomer based on the actual resources consumed.
For e.g a company sales some product to the customer and have the sales order for that product but sometimes customer requires installation service so you have to send your technician at customer's place to do the installation. In this case cutomer is required to pay for material + installation charges + travelling charges etc.
Now the material price is in sales order and travelling + installation charges will come from service order.
So you have to create billing request in DP91 for installation and travelling charges. Then in the final invoice combine the sales order and billing request.
The configuration steps required for this are as follows.
1. Time Entry for the Projects through CATS
2. Assignment of Activity type rate in KP26
3. Proper DIP profile configuration
4. Condition record in VK12