The case involving one system is not essentially different from the case involving more than one system. Both the ordering and the supplying plant exist in one APO as plant locations. Where there are separate APO systems, both plants only exist as plant locations in their own APO system.

The deployment run in APO generates purchase requisitions (PReqs) that are converted into purchase orders in the R/3 System. From the materials planning view, the PReq in APO is a stock transfer between two plants. It has a requirement in the supplying plant and a receipt element in the ordering plant. At this point, the vendor number is not known in the APO system. This is determined in the ordering R/3 System from the supplying plant. The PReq is visible in the ordering R/3 System but not in the supplying R/3 System.

As an alternative to deployment in APO, you can also create Preqs manually in the R/3 System. You can carry out sourcing in APO for these PReqs:

Using rule-based ATP in APO allows you to assign the supplying plant according to the ATP situation. The plant where the goods are available is assigned as the source of supply. This process can be summarized as follows:

1. Determination of the source of supply in the R/3 system determines at least one vendor that is managed in the APO system as a plant location, or a "real" supplying plant that exists in the same R/3 System. If the system can derive a supplying plant from this, the system then performs the ATP checks for this supplying plant. If the system cannot determine a source of supply, but the material is known in the APO system, the system uses the ATP controller to call a record. This record contains the ordering plant. ATP rules are stored in APO for the ordering plant. You can use the ATP rules in APO to control whether the locations are determined immediately.
2. APO performs an ATP check according to the defined rules. The checks are mainly interrupted when the system finds a location where the material is available. You can use a customer exit to perform a further search until the system offers you a list of locations.
3. The "normal" sources of supply are added to those in the APO system. If only one result is found, then this source of supply is assigned. Where there is more than one resulting source of supply, the customer exit will run. In this customer exit, you can define a logic different from the APO rules in order to determine the source of supply. If the result is still not definitive, then the system displays a standard dialog box for selecting the source of supply. The confirmed date is displayed in the Realistic Delivery Date field. The confirmed quantity is copied as an additional column.
4. If there is only partial availability, then the system does not perform a split in the PReq item. This is because the PReq does not recognize any schedule lines.

When the purchase order is created, the purchase requisition is deleted in both the R/3 and the APO systems. Instead of the PReq, there are now two separate objects; a purchase order and a delivery. In APO, the purchase order represents the receipt element in the ordering plant, and the delivery represents the requirement in the supplying plant.