W9
The W9 form provides us with your US tax ID or Taxpayer Identification Number (TIN) for reporting purposes. A financial institution or person might use this to report certain information about you, including income, contributions to IRAs made by you, interest, dividends and capital gains earned by you, certain real estate transactions entered into by you, the balance of certain foreign financial accounts, etc. to the IRS.
Form 8938
FATCA (or the Foreign Account Tax Compliance Act) came into law on July 1, 2014. FATCA requires non-US foreign financial institutions (FFIs) and non-US non-financial entities (NFFEs) to identify and disclose their US account holders and members or become subject to a new 30% US withholding tax for any payment of US source income and proceeds from the sale of equity or debt instruments of US issuers.