Ok, what if the letter at the end of the ID card is opened by SMS?

When the ID number is followed by an X, it is usually written as an English capital letter "X"; When doing business by telephone, because the telephone keyboard has no letters, it is generally replaced by "*".

if you buy it online, you have to enter letters where they are, and they cannot be replaced. It is recommended to use uppercase. If it is not indicated on the general website, it should be recognized in both uppercase and lowercase.

So, it can be seen that "X" is not an English letter, but a Roman numeral. So it has nothing to do with case. The correct spelling is: X.