Why do I need to mail Form 8453 separately when e-filing Federal?

[Asta1]

I'm e-filing my federal, why do I need to mail form 8453 along with form 8949 separately?  Shouldn't both forms be in the e-filing package already?  

[RobertB4444]

Form 8453 is designed to submit supporting documents that need to be sent to the IRS for your tax return.  You efile the return and then mail in 8453 along with whatever supporting documents you need.  The system thinks you need to send supporting documents for some reason.