It says I maxed out the federal $10,000 deduction for itemized state and local taxes, but I didn't enter that amount. Where does it come from?

[Ashby]

Your itemized deductions for state and local taxes include:

• Real Estate Taxes
• Personal Property Taxes (ex. car tax)
• State Income Taxes

Most likely, your state withholdings on your W-2(s) and real estate taxes from your 1098 mortgage statement maxed out the $10,000 limit. 

Last year's tax reform introduced a new $10k limit on these deductions. In years 2017 and earlier, you were not capped.