It's best explained by example: if you originally bought 100 shares for $1000, your cost basis was $10/share. You receive a non dividend distribution in box 3 of form 1099-Div. of $100, which you do not report on your tax return.
Your cost basis in your 100 shares is now $900 ($9 / share). You make that adjustment, in your own records. It does not go on your tax return, TurboTax will do nothing with the amount in box 3 of the 1099-Div.
