Since the information from your Form s 1099-B would normally have been reported on Form 8949, check the Form 8949 box.
According to E-file in the Instructions for Form 8949, you can either attach Form(s) 8949 showing all the individual transactions to Form 8453, or you can attach one or more statements containing all the same information as Form 8949, instead of attaching Form 8949, if the statements are in a format similar to Form 8949.
If all of the information from your Forms 1099-B that would normally be reported on Form 8949 doesn't appear individually on the Form 8949 that you e-filed with your return, you'll need to send supporting statements that reflect the individual transactions to the IRS with Form 8453. That is, copies of your Forms 1099-B.
The IRS needs to see each transaction separately so that it can match up information it’s received from each of the reporting institutions.
