Form 8621 is filed by a U.S. shareholder who owns stock in a foreign investment company. The form is used to report income, make an election to extend the time for payment of tax, and to pay an additional tax and interest amount. The IRS uses Form 8621 to determine if these shareholders have correctly reported amounts of income, made the election correctly, and have correctly computed the additional tax and interest amount.
The latest form for Information Return by a Shareholder of a Passive Foreign Investment Company or Qualified Electing Fund expires 2021-12-31 and can be found here.