SAP

Assuming you have defined split valuation types in the sap img, you must remove all stock for the current period and one previous period before you can create the material with different valuation types.

The best method is to wait until the last day of a period, and then perform a physical inventory adjustment to remove the stock.

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In normal materials management there are basically 2 basic inventory management systems, the P system and the Q system. 

P system is a system wherein replenishment is done keeping the quantity constant. The period becomes the variant. In other words you fix the quantity you want the stock to dip, to trigger a requirement. As soon as the stock level is reached you replenish the stock. During a lean period the time taken to reach the level will be longer and during an active period the time taken to reach that stock could drop.This normally relates with your consumption based planning. 

Q system is a system wherein replenishment is done keeping the period constant. The quantity becomes the variant. In other words you will check for the level of stock at fixed time intervals (daily, weekly, monthly etc.) compare it with the requirements on that day and trigger the procurement process for replenishment.This normally relates with your MRP.

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