The phrase "last digit" refers to the final number or numeral of a larger number. It is the digit found at the rightmost position in a number. Full definition
This is usually the 4 last digits of your telephone number, and this, too, may be changed later. (hardinglibrary.org)
It involves the surgical removal of the entire last digit of each toe or amputation comparable to the removal of human fingertips to the first knuckle. (cap4pets.org)
It's gotten so bad that the government mandates, by law, people can not drive their car for two days out of the week depending on the last digit of their license plate. (fortune.com)