Well I'm filling out all the goodies that is required and I come to the W9 Tax Form at IB and I'm clueless to what being "exempt for backup withholding" is when it gives me the option to select it or not. I've even read through the link which gives you more information on what it means but I must admit my taxes are straight forward and I do lack sophistication when it comes to tax issues.
Formerly, I was a Financial Planner... hopefully my understanding on this is current. "Backup Withholding" is like a proactive measure by the IRS for those who have formerly underreported income and then not been able to pay the taxes. After such an episode/violation, the perp has to have income tax automatically withheld by the institution and declare such on W9. If you'd ever been involved with the IRS in such a way, I'm sure you'd remember. If not, you're unlikely to be subject to backup withholding, but I believe you have to sign the W9 declaring that you are not subject to it. Hope that helps.