Stock Items are purchased or sold on the basis of quantity. The quantity is measured by Units. Hence, it is necessary to create Units of Measure. You can have simple units such as numbers, meters, kilograms, and pieces or compound units like box of 10 pieces [1box =10 pieces]
Go to Gateway of Tally > Inventory Info. > Units of Measure
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Creating a Simple Units of Measure
Creating Compound Units of Measure