If you have not been told by the IRS in a formal letter that you are subject to back-up withholding, then you should answer that you are not subject to back-up withholding when filling out the W9 form. Those people who have been told that they are subject to back-up withholding have bee. understating their income regularly (by not declaring all income on the income tax return) or have previously given an incorrect Social Security Number or Employer Identification Number to another payer. So, if you answered Yes, I am subject to back-up withholding on the present W9 (whether truthfully or not), the payer must withhold income tax (24%) from all money paid to you. People subject to back-up withholding don't get a tax bill at tax time from the IRS as another answer claims; instead, they get only 76% of what is "owed" them by the payer, and if they don't declare any of this money (neither the income nor the tax withheld) on their tax return, at least the IRS has gotten some tax revenue from them. As dwizum points out in a comment, regardless of how you answered the question on the present W9 form, if the website cannot validate the Social Security Number or Employer ID number that you have so thoughtfully provided them, then the website will withhold 24% from all payments it makes to you even though you have claimed that you are not subject to back-up withholding.