On Lottery Post they are called "Sum Last Digit", because that is a clearer explanation of the concept. "Short Sum" does not really make sense.
I also wouldn't refer to it as a "root" because to get a root you would add the digits in a sum together repeatedly until you are left with only one digit.
To get the Sum Last Digit (Short Sum) you calculate the sum of the digits, and then the Sum Last Digit is the right-most digit (the "ones" column).
For example, with the combination 789, the Sum is 24, so the Sum Last Digit (Short Sum) is 4.