Notice of Deficiency
A "notice of deficiency" is a formal letter from the IRS stating that you owe additional tax and giving you 90 days to challenge. The deadline is important because after this, the taxes can no longer be legally disputed.
The first page of the notice of deficiency will set out the proposed tax and the deadline to challenge.
Please feel free to contact me for questions about your notice of deficiency (and whether you have a case for challenging it and reducing the tax).