Go to SPRO -> SRM Server -> Cross - Application Basic Settings -> Define Objects in Backend System
Fill the table with Purchasing Group, Product Category, Backend and the action that should perform the system, at this point we have three options :
Always Reservations form material types subject to inventory management.
Reservation generated if stock available. /Otherwise External Procurement.
Always External Procurement
Choose the options according to your business requirements and save.
At SRM Server, Implement BADI BBP_EXTLOCALPO_BADI Method DETERMINE_EXTPO in order to start working in Classic Scenario. This Badi is used to switch between Classic and Extended Classic scenario in runtime, the Badi allways assume Classic until BBP_EXTPO_GL-BBPEXPO = ‘X’.
At SRM Server, Implement Badi BBP_TARGET_OBJTYPE Method DETERMINE_TARGET_OBJECT_TYPES and according to your business requirement write the code to choose when to create a Reservation, Purchase Requisition or Purchase Order as follow-on document, To indicate with document will be created in Backend use the field CT_ITEM-PACK_OBJTYPE and assign the BUS value corresponding to the document you want to create, in case of reservation , pack_objtype = 'BUS2093'.
At Backend Server, you must create number range for reservation or any other document you want to crate. In ase of reservation I will use RB.
At SRM Server, Assign Number Range to document type in SPRO->SRM Server-> Number Ranges