Purchase Requisition 
A purchase requisition is a request or instruction to Purchasing to procure a certain quantity of a material or a service so that it is available at a certain point in time.
The system allows you to maintain instructions to procure a certain quantity of different stock segments of a material.
When you manually create a purchase requisition, the system allows you to enter a valid stock segment for a material. During MRP if the required quantity of stock is not available, the system proposes a purchase requisition and determines stock segment for the material. The system determines the stock segment based on the segmentation strategy you assigned for the material of the plant.
When you create purchase orders with reference to a purchase requisition, the system allows you to change the stock segment and ordered quantities for a material that are copied from the reference document.
The system determines the requirement segment for a stock transport requisition based on the stock segment and the segmentation strategy you assigned for the material in the material master.
Structure
On the Item Overview screen, you can specify the following information for different stock segments of a material:
Purchase requisition quantities
Delivery date
The system also displays the schedule line number and ordered quantities.