Not quite. It is a separate tax that is only assessed on capital gains above the threshold. It's a little hard to put into words so maybe some examples would help:
You are married with $150,000 ordinary income and $150,000 capital gains. $100,000 of your capital gains would be taxed at 15% and $50,000 would be taxed at 18.8%.
You are married with $250,000 ordinary income and $50,000 capital gains. In this case, the entire $50,000 capital gains would be subject to both the 15% capital gains rate and the 3.8% NIIT.
So depending on the source of your income, you may pay the NIIT on all your capital gains or only a portion of them. It does not quite fit neatly on the capital gains tax rates tables that you see everywhere, although you might find it somewhere in the small print.
