Purchase requisitions can be created in three ways:
Manually by individual users
Automatically via the MRP run
Automatically via any logistics order (production order, enterprise asset management order, and many more)
During the creation, a source of supply can be assigned, either manually or automatically. These purchase requisitions can be converted in different ways.

SAP S/4HANA provides the following functions for converting assigned purchase requisitions to follow-on documents:
Assignment list
Manual conversion in the Process Purchase Requisitions app
Add from Document function in the Manage Purchase Orders app
Automatic generation of purchase orders