This article is not about issue or solution, it is more on a overview of M3's MRP, so that you can understand it better. 

Material Requirements Planning (MRP) is a mode of planning that plans requirements in order by the lowest level in which the item appears ina bill of material. That is, it plans all end items first, then all items at the next level, and so on, backward planning each requirement from therequirement's needed date to the item's lead time and batching together requirements needed at the same period of time.

Unlike APS, MRP does not consider routing times, resource capacity, or shifts. The responsibility rests with you, the planner, to make sure your shop floor has the available capacity to work the plan. MRP generates planned orders and exception messages to help you implement an accurate plan.

This information is used in MRP planning:

• Low-level Codes and Bills of Material:

Each item has a low-level code that represents the lowest level at which the item appears in all current, job, and production schedule bills of material (BOMs). An end item always has a low-level code of 0. Items in the next level of the BOM have a low-level code of 1, the next level is 2, and so on. MRP processes items in order by low-level code, and processes an item only at its lowest-level code. 

• Routings:

MRP does not directly factor operation duration times into its planning logic. 

• Item Lead Times:

MRP uses the item lead time to determine the due dates on planned orders it creates to satisfy requirements, planning backward in time from the date the item is needed. 

• On-hand Inventory and Receipts:

MRP calculates the on-hand inventory quantity at the beginning of the process as the total on-hand quantity (that is, on-hand - QtyReserved for customer orders) at all nettable stockroom locations across all non-dedicated-inventory warehouses at the site. Receipts (also called "planned supplies") include expected incoming quantities from jobs, production schedule releases, MPS receipts, purchase orders, and transfer orders. 

The system considers on-hand inventory and receipts in determining the net requirements for an item. Transit locations must always be non-nettable for site-to-site transfers. Cross-site transfers show up as receipts in the To Site, so the transit location has to be non-nettable so that the quantity isn't considered twice by MRP; in the beginning projected on hand and as a receipt. When performing warehouse-to-warehouse transfers within a site, the transit location used by those must be nettable.

• Independent Requirements:

An independent requirement is a demand for an item that does not originate from another requirement. Forecasts, customer orders, and demand transfer orders are independent requirements. MRP starts by planning independent requirements and generates "dependent requirements" for the next levels in the item's BOM.

• Master Production Schedule:

The master production schedule (MPS) allows you to control production of key end items to help you protect your schedule from fluctuationsin order-based demand (forecasts, customer orders, parent job orders, etc.). MPS is a manually created, anticipated build schedule for anitem. You create it based on your expectations of demand and your estimation of resource capacity.

When you run the MPS Processor, planned orders are created for MPS item requirements that are due outside the MPS plan fence, but notfor the components in the MPS item's bill of material. When you run MRP after running MPS, MRP does not create planned orders for the MPS end item, but does pass those requirements down to the components in the MPS item BOM, creating PLNs for the components.

Information that MRP Generates

When you run an MRP process, the system generates planned orders (PLNs), as illustrated in the example below. PLNs represent MRP's suggestions of how you can manufacture or purchase the item to satisfy the requirements. A PLN is not tied to a particular requirement. You must examine each PLN and firm it into a real job order, purchase order, transfer order, etc., as you deem appropriate.