Count Unique with Criteria

Hi,

I am looking to count the number of unique people that fit a couple of criteria. I found a solution online, adapted it to my needs, and it works, but I don't completely understand it. It uses SUM IF FREQUENCY IF MATCH ROW. Could someone explain what's going on here? I use table formulas in my solution as I think it's clearer but feel free to change to regular.

I attached a sample spreadsheet with the solution that works.

Thanks in advance,

David