Inventory Records, Lead Times, and Item Master Data
Item master data
Every planning decision depends on the item master: item number, description, unit of measure, item type (make, buy, phantom, reference), planner or buyer code, default lot size, safety stock, lead time, and ABC class. A wrong UOM or a missing planner code produces plausible but wrong planned orders. Buy items carry a purchasing lead time; make items carry a manufacturing lead time built from queue, setup, run, move, and wait. Rolling these up through the BOM gives cumulative lead time, which sets the minimum planning horizon.
The inventory record
On hand, allocated, and available are different numbers; MRP nets against available. Scheduled receipts are open orders with due dates; planned orders are MRP's proposals. Each period: projected available = prior projected available + scheduled receipts + planned order receipts − gross requirements. If the result breaches safety stock, a net requirement is generated.
Safety stock and safety lead time
Safety stock is planned as a requirement, usually in the first period, or as a floor the projected available must not breach. Safety lead time protects dates by shifting orders earlier. Safety stock protects quantity; safety lead time protects timing.
Worked example
Item X: on hand 40, safety stock 10, lead time 2 weeks, lot-for-lot. Gross requirements 30 in week 2 and 30 in week 3. Projected available: week 1 = 40; week 2 = 10; week 3 = −20, below the floor of 10, so the net requirement is 30 (enough to restore 10, not 0). Planned receipt week 3, release week 1. Treating the floor as zero understates the order by 10.
Traps
Netting against on hand rather than available. Forgetting the safety-stock floor. Assuming lead time is fixed when queue dominates - chronic overload shows up as growing queues, not as an updated master-data field.