It seems to be when an incorrect number is entered and it takes you to the selection screen. The default is the first person in the alphabet and if you accidentally choose select instead of back the purchase is charged to the default person. It would seem to me that a better option than that would be when an incorrect # is entered you would get a message on the screen that lets you know that the # is incorrect. If you want to look up by person you can choose the customers tab in the lower left of the POS screen to select by name.