This may refer to the following: Gain or loss(-) and Loss not allowed
• Your gain or loss for each sale will be listed here, but will not be reported to the IRS by TD Ameritrade. If you have wash sale adjustments listed in column 1g, these have already been accounted for in determining your gain or loss for that transaction.
I presume you sold some security short and subsequently covered the short at a loss. You should add that manually to the 1099 - B in TurboTax. Even though TD Ameritrade doesn't report this, you will still need to report all realized profits and losses.