Why do I have to mail form 8453 when my return already includes a fully itemized 8949?

[deathtoll2001]

I just read https://ttlc.intuit.com/questions/3840518-turbo-tax-says-i-must-mail-form-8453-8453-wants-me-to-atta..., but still have a question:

I imported my investment forms from Wealthfront and Betterment, and so TurboTax generated a form 8949 with fully itemized sales.

Apparently, I also paid some foreign taxes as part of what they did on my behalf.  So far as I can tell, by selecting the "foreign tax credit" option instead of the "federal itemized deduction" for how to report this, I now need to mail in a form 8453 with an 8949 supporting document clause.

What do I actually need to mail in as supporting documentation?  Do I print out the full 1099-combined forms I got from both firms?

[KittyM]

You will need to mail copies of your 1099-B's and the Form 8453.  I recommend you black out the account number.