A control number, it's sometimes used by companies internally. You really don't need it
If you get errors when attempting to e-file with an empty Box D,
just enter any number in this format: 5 digits, space, 5 digits (for
example, 12345 67890).
On the other hand, if you're trying to import your W-2 with a blank
Box D, click skip import and then manually enter your W-2. You won't be
able to import using the 5 digit + space + 5 digit solution for
e-filing.
