DLP is a mechanism for preventing data leaving the organization where that information is likely to contain confidential data, such as credit card numbers or national insurance numbers. In addition, you can create custom rules that cover custom information, such as employee numbers or other definable data. Exchange Online implements this protection by matching the format of blocks of numbers to that in the specified rule. An example of a rule might be US Financial, which applies the formats for 'Credit Card Number' or 'U.S. Bank Account Number' or 'ABA Routing Number'. A message with a number such as 1234 5678 9012 3456 or 1234-5678-9012-3456 would then be picked up.