K is for the pre-tax dental and vision insurance deduction amount. This is the amount you had withheld during the year to pay for your dental and vision coverage. It is a reporting number only on the W2 itself and not used in calculating taxable wages. The amount reported with a code K has already reduced the taxable income you have reported in box 1 taxable wages of the W2 form, so you should not be using this amount as a part of your qualifying medical expenses for the tax year (if you are itemizing deductions).
