[UPDATED JANUARY 2017]
Form W-2 (wage statement) Box D is called the Control Number field. It is usually located below or near the Employer's Name and Address.
The Box D Control Number is a code that uniquely identifies your particular W-2 document in your employer's records. This number is assigned by a company's payroll processing software, and is needed if attempting to electronically import your W-2 into a tax software program (like TurboTax). Please note that not all W-2s (or employers) will necessarily include or support a Box D Control number. Your alternative to importing a W-2 document is to simply input the information manually.
The actual Control Number itself is not necessary in completing your tax return, unless you wish to import your W-2 into TurboTax (or other tax software); and it that one instance it is required. In other words, the W-2 Control Number, whatever it is, will never increase or decrease your actual tax refund (or your amount owed) by a single penny.
If you choose to type in your W-2 manually, rather than importing it electronically, then you should not have to enter anything from Box D at all, if you do not want to. In fact, you can simply leave the field blank. The exceptions to that would be if you are attempting to e-file your return and see some sort of W-2 Control Number related error; or if the program insists on a value before allowing you to continue. In those cases, we have a TurboTax FAQ article that can help. We've linked this answer to that FAQ page below.
We hope this information is useful to your needs, and thanks for asking this important question.
https://ttlc.intuit.com/replies/4417180
