Perhaps IRS form 8453 was prompted by the last option listed on the form.
Form 8949, Sales and Other Dispositions of Capital Assets (or a statement with the same information), if you elect not to report your transactions electronically on Form 8949.
Did you enter a summary of your brokerage sales into TurboTax?
Form 8453 is mailed to the IRS within three business days after receiving notice that the IRS as accepted the electronically filed tax return. The IRS can then compare the brokerage statement against the the summary that was reported.
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