Section 1446(f) regulations impact foreign customers (W-8 customers) who have accounts with transactions in securities that fall within the scope of Section 1446(f). This regulation specifically relates to the taxation of certain foreign partners in partnerships conducting business in the United States.
It's important to note that Section 1446(f) regulations do not impact United States domiciled customers (W-9 customers). Only foreign customers with accounts and transactions involving securities covered by Section 1446(f) are subject to these regulations. The specific list of securities covered by Section 1446(f) can be found [here].