"Form 1099-R: Source Form should have a box checked" What does this mean?

TurboTax is checking my federal entries. The following entry error and options came up:

Check this Entry

Form 1099-R (Charles Schwab & Co Inc): Source Form should have a box checked.

_ No Entry

_ 1099-R box

_ CSA-1099-R box

_ CSF-1099-R box

_ RRB-1099-R box

What does this mean? The form was downloaded from Charles Schwab. I don't see this anywhere on Form 1099-R. What is TurboTax asking? What is a source form? Should I just check "1099-R box?" 

Thanks for your help!