A more thorough explanation:

A deficiency assessment is a determination by the IRS of additional tax owed by a taxpayer who underpaid. This assessment is made after administrative review and tax-court adjudication.

For example, if a taxpayer fails to report all of their income on their tax return, the IRS may conduct an audit and determine that the taxpayer owes additional taxes. The IRS will then issue a deficiency assessment to the taxpayer.

Another example is if a taxpayer claims deductions or credits that they are not eligible for, the IRS may determine that the taxpayer owes additional taxes and issue a deficiency assessment.

Overall, a deficiency assessment is a way for the IRS to collect the correct amount of taxes owed by a taxpayer who underpaid.