The IRS is able to compare the enclosed brokerage statement against the summary that was reported on your tax return. We can only presume that this process is more workable for the IRS than comparing your tax return to its1099-B master file.
See IRS Form 8453 and 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.
IRS 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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