If you have 100 red bricks and want to sell them in batches of 10, select 100 as Quantity and 10 as Bulk. Buyers can then buy this item only in multiples of the bulk amount. The default for Bulk amount is 1.
The buyer can buy the remainder of the stock if there is more stock than quantity divisible by bulk amount. For example if quantity is 21 and bulk amount is 5, the buyer can buy the item in quantities of 5, 10, 15, 20 or 21. If the bulk amount is higher than the quantity available then the buyer can buy the quantity available.