A scheduling agreement is a long-term agreement with a vendor for the supply of materials or services at predefined dates and quantities. To enable classic MRP to automatically create schedule lines in case of material shortage, you must do the following:
Add the agreement to the source list and mark it for MRP relevance. The source list is a list of possible sources of supply for a material, such as vendors, contracts, or scheduling agreements. By adding the agreement to the source list and setting the MRP indicator, you specify that the agreement is a valid and preferred source of supply for the material.
Set the creation indicator for delivery schedule lines on the initial screen of the MRP run. The creation indicator determines whether the MRP run creates procurement proposals, such as purchase requisitions, planned orders, or delivery schedule lines. By setting the creation indicator for delivery schedule lines, you instruct the MRP run to create schedule lines for the scheduling agreement, based on the material requirements and the agreement conditions.
Adding the agreement to the quota arrangement and marking it for MRP relevance is not necessary, as the quota arrangement is used to distribute the total requirements of a material among several sources of supply. Setting the creation indicator for purchase requisition on the initial screen of the MRP run is not correct, as the purchase requisition is a different type of procurement proposal than the delivery schedule line. Reference: Scheduling Agreement | SAP Help Portal, Source List | SAP Help Portal, MRP Run | SAP Help Portal.

The master data fields that can have an impact on whether MRP creates planned orders or purchase requisitions for a material are:
MRP type: The MRP type is a parameter that defines how a material is planned by MRP. Depending on the MRP type, MRP can create different types of procurement proposals for a material. For example, if the MRP type is PD (MRP), MRP can create either planned orders or purchase requisitions, depending on the procurement type of the material. If the MRP type is VB (Reorder Point Planning), MRP can only create purchase requisitions, regardless of the procurement type of the material.
Procurement type: The procurement type is a parameter that defines how a material is procured, either internally or externally. Depending on the procurement type, MRP can create different types of procurement proposals for a material. For example, if the procurement type is E (External Procurement), MRP can only create purchase requisitions for a material. If the procurement type is X (Both Procurement Types Possible), MRP can create either planned orders or purchase requisitions, depending on the source of supply determination.
The master data fields that do not have an impact on whether MRP creates planned orders or purchase requisitions for a material are:
MRP groups: The MRP groups are parameters that allow you to group materials with similar planning characteristics and assign them common MRP settings, such as planning horizon, lot-sizing procedure, or planning calendar. The MRP groups do not directly affect the type of procurement proposals created by MRP, but they can influence the quantity and timing of the requirements and receipts.
Material type: The material type is a parameter that defines the attributes and properties of a material, such as valuation class, price control, or account determination. The material type does not affect the type of procurement proposals created by MRP, but it can influence the valuation and accounting of the material movements.

A work center hierarchy is a business object in PP/DS that defines the structure and sequence of resources that are used for a production process. You use this object to represent the logical and physical dependencies of the resources and to control the scheduling and sequencing of the operations or orders. A work center hierarchy allows you to model the production flow and capacity more realistically and ensure the technical feasibility of the production plan1 A work center hierarchy can also be used to cumulate the available capacity and capacity requirements of the resources in capacity planning. You can cumulate the available capacities from subordinate work centers to a superior work center level in order to determine the total available capacity for the superior work center. This way, you can use a work center hierarchy as a group of alternative work centers that provide a cumulative available capacity for production23 Reference:
Work center hierarchy | SAP Help Portal
Work Center Hierarchy - SAP Help Portal
Highlights for Manufacturing in SAP S/4HANA 2021 - Part 2: Production Planning, Engineering, Operations | SAP Blogs

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.