IRS Form W-8 is used to certify your status as a non-U.S. taxpayer. It is also used to claim reduced tax withholding on dividends and other income paid with respect to U.S. securities.
Without proper documentation, brokers may be required to impose exceptional withholding tax on transactions occurring in the account and/or report the account as non-compliant to tax authorities.
Find a detailed description on how to Update Tax Forms in the Client Portal Users' Guide.