On the "customer search" option, what would be the "customer ID"?
The Customer ID will assign an ID to customers that are repeatedly running transactions. It will match transactions by gathering information such as a card information, first and last name, address, etc... This option will give you a statistics on how often they buy from you.
You can also view the total dollars spent by the customer, a purchase frequency information, refunds, etc... This is useful when creating promotional materials for credible customers, and put restrictions on customers with previous suspicious behavior.