On an overnight flight from Boston to Chile 4 or 5 years ago, I ran into a situation I hadn't encountered previously. My seat wasn't great and the flight was perhaps half full, so I tried shifting to an empty seat in a block of 4 where there was only one other person. I was told by the flight attendant that I couldn't sit in that row because the person there had somehow "reserved"* those seats in addition to their paid seat. My wife encountered the same thing in another row. It was clearly a regular occurrence on that flight. We were told by another person on the flight that travel agents can do this for you. I never investigated further, but I am curious to know how to do this if an opportunity should arise in the future. Is there a name for this practice?
If memory serves, it was an American Airlines flight, but I'm not 100% certain of that.
*I don't recall the exact term the attendant used.