They are using the "rule of nines" (i.e. the sum of the digits, perhaps iterated). It is very handy when you have the decimal representation of the number to operate on paper, but it's not as handy when you have the numbers in the computer. I "translated" that to "modulo 9" because it is equivalent and is easier to understand for not native English speakers. [Hi from Argentina!]

Also, they are using the last digit of the number, that I translated to "modulo 10".

Another reason for the translation is that the mathematical structure is more clear in "modulo 10, modulo 9 and modulo 24" than in the version "last digit, nines rule and modulo 24".

In a computer is much more efficient to use modulo that transforming the number to a string and then operating with the digits, so I hope they have an efficient implementation but it is not clear from the pdf.