Before you run MRP, you set the MRP planning horizon. This specifies a period of time, in calendar days, over which MRP is to plan. MRP only processes material requirements within this horizon. The longer this horizon, the longer it takes MRP to plan. However, the planning horizon should be at least as long as the longest cumulative lead time, plus any associated preparation times. Usually, the master schedule also covers a time frame that is at least this long.

Cumulative lead time represents the longest planned length of time to obtain an item, assuming that neither it nor any of its components (if any) are in stock. For a manufactured item, this includes the time it takes to acquire raw materials, inspect them, make any subassemblies or components, and assemble and inspect the finished product. These times are entered in Item Planning Maintenance for each item in the fields Mfg LT, Pur LT, and Ins LT.

To provide visibility for what orders are due to be released, MRP generates Release Due action messages. Typically, you want to see these in advance, not just on the day the order is due for release. You can set the release horizon for any number of days. MRP generates Release Due messages only for orders due to be released within this number of days from today’s date.