The presence of uniformed officers acts as a deterrent to shoplifting activity and they are mostly used by high-end retail establishments. Shoppers in some stores are asked when leaving the premises to have their purchases checked against the receipt. Some expensive merchandise will be in a locked case requiring an employee to get items at a customer's request. The customer is required to purchase the merchandise immediately, or it is left at the checkout area for the customer to purchase when finishing shopping. Many stores also lock CDs, DVDs, and video games in locking cases, which can only be opened by the checkout operator once the item has gone through the checkout. Some stores will use dummy cases, also known as "dead boxes", in which the box or case on the shelf is entirely empty and the customer will not be given the item they have paid for until the transaction has been completed, usually by other store staff.