A more thorough explanation:

A notice of deficiency is a letter sent by the tax administrator to a taxpayer if they find that there is a shortage of income tax in the taxpayer's tax return. The notice is usually sent by mail and includes information about the taxpayer's right to contact a local office of the taxpayer advocate and the location and phone number of the appropriate office.

For example, if a taxpayer files their tax return and claims a deduction for a business expense that is not allowed by the tax code, the tax administrator may send a notice of deficiency to the taxpayer. The notice will inform the taxpayer of the deficiency and provide instructions on how to dispute the deficiency.

Another example is if a taxpayer fails to report all of their income on their tax return. The tax administrator may send a notice of deficiency to the taxpayer, informing them of the deficiency and providing instructions on how to resolve the issue.

These examples illustrate how a notice of deficiency is sent to a taxpayer when there is a discrepancy in their tax return. The notice provides the taxpayer with information on how to dispute or resolve the deficiency.