1. Service Tag or ST is 7 alphanumeric characters long. To avoid confusion, there are no letters E, I, O, U.
2. Express Service Code or EX is 11 digits long, it acts as a replacement for service tag when shared over a phone call, as spelling letters is not required.
3. S/N is a long 20-28 alphanumeric identifier, typically used for monitors with or without service tag.