Purchasing's hardest question is always the same: what should I order, how much of it, and by what date at the latest. In most factories the answer comes out of a foreman's experience, the production manager's guess and an Excel file kept on someone's desktop. This method carries the business for a while, but it turns out expensive in both directions. On one side the line stops because a single ancillary material was ordered too late; the team then brings it in on an emergency order, at a high price and sometimes by air freight. On the other side, raw material bought in excess on the reflex of never running short again piles up in the warehouse and cash is locked into stock. Both situations come from the same gap: the requirement was never calculated, only estimated.
What is missing is not really software, but the data the calculation rests on. Material requirements planning (MRP) is built on the bill of materials (BOM) and on lead time data. On the shop floor, however, the BOM is usually out of date; the product has changed, the recipe has been updated in someone's head but never written into the system. Scrap rates are not recorded, the alternative material is knowledge only the buyer holds, and lead times shift according to whatever the supplier promises. That is why in many companies the MRP module in the ERP does exist but is never run: it was tried once, the proposal list it produced did not match reality, confidence was lost and the file was closed. The source of the problem is not the module itself, but the data feeding it.
At its core the MRP calculation is a simple chain. Sales orders and the production plan give the gross requirement; the BOM explodes that requirement down to the component level. Stock on hand, open purchase orders not yet received and quantities reserved for other jobs are then deducted; what remains is the net requirement. In the final step the system counts back by the lead time and finds the latest date on which the order must be placed. Safety stock, minimum order quantity and lot sizing rules are layered on top. The output is not a list but a dated action plan: which material, in what quantity, from which supplier and by which day at the latest it must be ordered.
MRP II, that is manufacturing resource planning, adds the resource dimension on top of that calculation. The material may well be sufficient; but are the machine hours, the tooling and the shifts available to do the work? MRP II tests the plan for rough-cut capacity: which work centre is overloaded in which week, and can the delivery date you quoted actually be met. The boundary has to be drawn clearly here. MRP and MRP II answer the how much and by when question; which job runs on which machine, in what sequence and with which tooling changeover is a matter for advanced planning and scheduling, and that is separate work. Finally, an honest warning: MRP on its own does not reduce stock, and no calculation repairs bad data. If the bill of materials and the lead times are not correct, the proposal that comes out will be wrong too.