The letter explains the decision and informs you that the IRS has either accepted your return as filed, has made the agreed upon adjustments to your return, or has closed the review of your return because a Letter 3219, Statutory Notice of Deficiency, was issued and the 90-day period to file a petition with the United States Tax Court has expired.
If you find that it is necessary to make changes to your return after the IRS has completed its review, you may be able to file a Form 1040-X, Amended U.S. Individual Income Tax Return. If you’re requesting a refund of the tax you paid, the due date for filing that request is generally three years from the date you filed your original return or two years from the date you paid the tax, whichever is later.