I received a LTR 12c from the IRS stating they want any supporting documents verifying income and withholding. Does that include schedule C? Or only W-2 and 1099's?

You only need to send them your W-2(s) and 1099(s) and a copy of your Schedule C.  The other information will only be needed in case of further review and then the IRS would request it.

[Edited 03.05.18 / 12:13pm]

Please note I have updated my response.  Please review the updated response.  Thank you.

Thank you

I received a similar letter today and ill be sending the requested information including SS statement indicating the additional withholding. But I am confused as to why this information is needed if the IRS already has it. Any ideas?

The IRS could be verifying the amounts claimed or that  you are really the one who filed the return or something else. There are a lot of moving parts the IRS deals with so send what they want.

@tom sturm

Thank you for your quick response.  The irs told me they were too busy and to call back another time.