What about the ID number of Hong Kong ID card?

the number of the Hong Kong identity card is * * * six digits, plus the number in brackets is * * * seven digits, which is preceded by the English letter code.

This letter or number in brackets is actually a test to see if the previous letter or number is misspelled.

The first 7 digits of letters and numbers are given by a weighted calculation method, so the last digit is generally called a test number, which is used to test the accuracy of the first 7 digits of an ID card, such as Z123456(7) and Z1234567(a).