S2421HN, serial number discussion

i m curious , the serial number of a dell s2421hn monitor, is 20 digit number? or 23? because on the box are 20 digits (CN-0DTCPY-BOZ00-22Q-0SBL) and on the back of the monitor are 23 digits (CN-0DTCPY-BOZ00-22Q-0SBL-A00 ) .

A00 is a part of the SN? or it is another thing?

Which one is the serial number? thanks.