Final answer:
A periodic inventory system calculates COGS at the end of an accounting period, while a perpetual inventory system updates COGS continuously with each transaction.
Step-by-step explanation:
A periodic inventory system allocates cost of goods available for sale at the end of an accounting period, usually during the physical inventory count. This system does not keep track of inventory on a day-to-day basis. Instead, it calculates the Cost of Goods Sold (COGS) based on the cost of goods at the beginning of the period, adding purchases during the period, and subtracting the cost of the ending inventory. On the other hand, a perpetual inventory system allocates cost of goods available for sale continuously, as each sale transaction occurs. This system maintains a real-time record of the inventory levels, providing a more accurate and current valuation of COGS and inventory.