A. It stands for “Employment Security” and is the account number you use to file your quarterly unemployment taxes (box 6 on the 5208-A form). The format of the number is: 123456 78 9. If the number begins with 600, 601 or 602 and is formatted like this: 60x 456 789, it is the Unified Business Identifier (UBI) number, not the ES reference number. Q.