When calling a manufactured item a string of characters how might a random check digit be valuable?

Question

The first two characters are year: 06, the next is month made: H, A = January, the Third is Machine used (E or W), the last 4 digits are 1-1000. Someone says we should add a check digit behind the fourth sequence digit so that when you type it in later, you will get a "wrong" number if the improper digit is put in there. I am having trouble seeing the significance of how this might help. Can anyone help me see the light here?

Answer 1

Check digits ensure that the rest of the number/char sequence is correct. The last digit of a credit card is a check digit. The last digit of a UPC or ISBN is a check digit. They are essential to preserving data integrity.

Check digits are NOT random. They are calculated based on the other numbers in the string.