As IB LLC is a carrying broker domiciled in the U.S., it is required to report information and, in certain instances, make payment of withholding taxes to the U.S. tax authority, the Internal Revenue Service for all account holders. To certify oneself as a non-U.S. person, a Form W-8 is requested at the time of application and is required to be re-certified every three years thereafter. If IB does not receive the W-8 or the account holder fails to re-certify the W-8 in a timely manner, then the account holder is presumed to be a US person and, absent a W-9, may then be subject to back-up withholding taxes on interest, dividends and substitute payments in lieu, as well as gross proceeds.
By certifying yourself as a non-U.S. person through a properly completed W-8, your U.S. withholding is limited to dividends issued by US corporations. Note that virtually all countries apply withholding taxes when local companies seek to distribute dividends to externally based shareholders (whether those shareholders are corporate or not). The rate at which IB is obligated to withhold for a given payment depends largely upon whether there is a tax treaty in place between the country where the dividend paying country is based and the country of residence of the dividend recipient.