Form W-2 Box d is a Control Number. It is usually located below the Employer's Name and Address.
Box D: Is a Control Number. This is a code that identifies this unique Form W-2 document in your employer's records. This number is assigned by the company's payroll processing software. NOTE: Not all W-2's (or employer) will include a Box-D. If you choose, you do not have to enter anything from Box D. I hope this helps you. Thanks, have a great day!