How to fill in the last digit of social security ID card online?

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

If you buy it online, it is a place with letters. You need to enter letters and you can't replace them. Capitalization is recommended. If there is no indication on the general website, it should be both uppercase and lowercase.

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