For IRS underpayment penalty, what counts as "refundable credit" in 2018?

Question

The IRS states that in event of gross (>$1000) tax underpayment, penalties are avoided if current years withholdings and refundable credit is more than last years total taxes.

Nowhere in their explanation do they indicate to which Form 1040 lines do "total taxes" or "withholding and refundable credits" correlate, but I assume what they mean is that if, in Form 1040 , the value in line 74 (aka "total payments") of the current year is greater than value in line 63 (aka "total taxes") of the previous year, you are exempt from underpayment penalties.

First, am I correct in my understanding of this?

Second, now that Child Tax Credit is partially refundable for 2018, does it count towards the "refundable credit"? And if CTC does count towards "refundable credits" what is the applicable amount: $2000 or $1400 (refundable cap)?

Answer 1

A refundable tax credit is something that reduces your tax liability and if the tax liability is reduced to zero, then any excess tax credit (a.k.a. whatever is left over of the tax credit after your tax liability is reduced tp zero) is returned to you as a refund of excess tax payment. With a nonrefundable tax credit, the excess tax credit if any is not refunded to you. It is usually lost for ever, but in some rare cases might be held over and applied to the tax liability in future years.