Additional Child Tax Credit

What Does Additional Child Tax Credit Mean?
A refundable credit that can be claimed by taxpayers who are ineligible to claim the full non-refundable child tax credit, because it exceeds their total tax liability. The additional child tax credit was created to reimburse taxpayers for the non-refundable portion of their child tax credit.

The additional child tax credit is available to families with three or more qualified children.

Investopedia explains Additional Child Tax Credit
The additional child tax credit is calculated by adding 15% of the taxpayer's taxable earned income to any tax-free combat pay. The total amount that is in excess of $11,750 (subject to annual adjustments for inflation) is refundable. 

Even taxpayers with income below this threshold are eligible if they have at least three qualifying dependents and have paid Social Security tax in excess of the amount of their earned-income credit for the year. This credit is claimed on form 8812 and is also subject to the same phaseout limitations as the child tax credit.