The 'CL' command, or 'CLEAR' button, is used to clear any system messages that pop up, usually because the cashier has entered invalid input. I these cases, the message will instruct you to use this command to continue.
'CL' is also used to clear finished order. When an order is complete, enter 'CL' or hit the 'CLEAR' button and then begin entering new UPCs to create a new order.