Recent cost is updated when items are received into Inventory (or a Purchase Order is raised for an item). It is an information field only and is only used in a calculation if items are sold that are not in stock (inventory goes negative).
Inventory only keeps the total cost and total quantity on file and uses that to calculate unit costs when items are shipped. There are 5 FIFO buckets. If you receive 6 separate receipts, the final receipt will be averaged into the last bucket.
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Andrew Bates