periodic (regardless of lifo or fifo) uses the standard cost on the item card as cost of goods sold for the item. This cost is not changed unless a user revaluates it or changes it.
perpetual indicates the current cost of the item (depends on lifo or fifo) as your cost of goods sold. This changes constantly whenever a transaction is made against the item either manual, receipt, invoice match, etc etc.
look in your help file by pressing F1 in GP and look up "valuation" there's a really good explaination in there on what you're looking for.
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and they wonder why they call it Great Pains!
jaz