For example in decimal arithmetic, the digital sum of 123 + 789 is 802.
123 789 --- 802More usually the digital sum is calculated in binary where the result only depends upon whether there are an even or odd number of 1s in each column. This is the same function as parity or multiple exclusive ors.
For example:
011 (3) 100 (4) 101 (5) --- 010 (2) is the digital sum.