Modulo 11 is a method to calculate checksums or verification digits to numeric data. Its used extensively throughout the world in cases such as bankaccount numbers, creditcard numbers, ISBN numbers and the like. The basic idea is not as much encryption or something similar, but more a protection against human errors. By calculating and verifying the checksum you can quickly detect cases like changing a single digit, switching digits, missing a digit, etc. By verifying a number as early as possible, you can decrease the amount of rubish entering a business process, and hence ease operations.