That is just a number that the payroll team uses to identify your W-2 for their system purposes. Sometimes it may be blank since that is optional to them. You can skip that section and just leave it blank since that is not something the IRS will need.
Or if you are receiving an error when trying to e-file, just put any combination of numbers in this specific format 5 digits, space, 5 digits (for example, 12345 67890). See below for more details on the Control Number box.
