In your tax return you must have entered a summary of several investment sales. You have to send the details of the individual sales to the IRS. The list of the details is supposed to be in the same format as Form 8949. It does not have to be an actual Form 8949. You can probably just send a copy of the pages of your brokerage statement that show the individual sales. That's what most people do. Send a copy, not the original.
