Box 6 on Form 1099-R are Net Unrealized Appreciation (NUA) in Employer's Securities.
Net unrealized appreciation (NUA) is the difference between the original cost basis and the current market value of shares of employer stock. When those shares are distributed, ordinary income tax must be paid immediately on the cost basis of the shares of employer stock. So the amount in Box 6 is the basis that is being taxed at your ordinary income rate and should already be included in Box 1 on Form 1099-R.
When those shares are sold, you would then have a transaction to report as a Capital Gain/loss. That information will be provided to you on Form 1099-B with the details of the transaction.
