Backward scheduling is a method of determining the order dates for components from dependent requirements by starting from the requirement date of the finished product and working backwards through the production structure. Backward scheduling considers the following time elements:
Operation duration: This is the time required to perform an operation on a resource, such as a work center or a machine. Operation duration is influenced by various factors, such as the quantity, the lot size, the setup time, the processing time, the teardown time, and the operation scrap. Operation duration is subtracted from the requirement date of the finished product to determine the start date of the operation.
In-house production time: This is the time required to produce a material in-house, from the start of the first operation to the end of the last operation. In-house production time is influenced by various factors, such as the routing, the work center, the production version, and the scheduling margin key. In-house production time is subtracted from the start date of the first operation to determine the order start date of the material.
Backward scheduling does not consider the following time elements:
Total replenishment lead time: This is the time required to procure a material externally, from the creation of the purchase requisition to the receipt of the goods. Total replenishment lead time is influenced by various factors, such as the planned delivery time, the goods receipt processing time, the release time, and the scheduling margin key. Total replenishment lead time is used in forward scheduling, not backward scheduling, to determine the order dates for externally procured materials.
Planned delivery time: This is the time required to deliver a material from the vendor to the receiving plant. Planned delivery time is maintained in the purchasing info record or the material master record. Planned delivery time is a component of the total replenishment lead time, which is used in forward scheduling, not backward scheduling, to determine the order dates for externally procured materials. Reference: Backward Scheduling | SAP Help Portal, Scheduling | SAP Help Portal, SAP S/4HANA Production Planning and Manufacturing Certification Guide, page 77-78.

In SAP S/4HANA Advanced Planning (Production Planning and Detailed Scheduling, PP/DS), a production planning run (transaction /SAPAPO/CDPSB0 or heuristics) generates supply elements to cover demand.
Possible results include:
* Planned order(A): PP/DS creates planned orders (visible in /SAPAPO/RRP3) for in-house production to meet requirements (e.g., sales orders, dependent requirements). These are detailed with exact timings and can be converted to production orders later (e.g., via /SAPAPO/PROD).
* Scheduling agreement schedule line(B): For external procurement with a scheduling agreement (maintained in ME31L), PP/DS generates schedule lines (visible in ME38 or /SAPAPO/RRP3) to request delivery from vendors, aligning with precise scheduling needs.
Production order(C) is not a direct result-PP/DS creates planned orders, which are then converted to production orders manually or via a separate process (e.g., CO41).Purchase order(D) is possible in classic MRP for external procurement, but PP/DS typically generates purchase requisitions or schedule lines, not direct POs. This is per SAP's PP/DS planning output documentation.