Some part from Mr. Pongsuk's comments after recent interview with Nene Royal :-

If Nene wins AGT, she will be under contract with Simon Cowell’s company—the show’s owner— for one year (not forever, as some baseless rumors suggest).

It promises to be a fantastic experience, too, as she would get to perform three shows in Las Vegas (with a share of the revenue)— something Nene has always dreamed of doing; I can sense that passion in her.

But looking that far ahead might be getting ahead of ourselves... let’s focus on the immediate future.

Everyone is surely wondering: "Which song will she pull out to set the stage on fire in the finals?"

When we met, I asked her father directly, but he said they didn't know yet because the producers assign the song about two weeks before the show.

For the second round, getting "Hysteria" was a stroke of luck because she already knew how to play it.

Then, for the third round, she got "Black Hole Sun"; even though she hadn't played it before, she managed to rehearse it in time. (That’s part of her charm—interpreting a song so differently from the original that she delivers a performance that leaves the whole world stunned.)

I believe she should have received her assignment by this week... Western producers have a keen eye for this sort of thing—they’ve turned countless people into stars—and finding the "right song" is crucial for Nene’s performance.

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**My opinion:** Now we have more insights. For QF round, AGT's producer may even know she will receive a golden buzzer (nothing is random nor a fluke), therefore they told Nene to practice for a song quite remote to general pop-based US viewers. But on the finals the producer may now allow she to sing a MORE POPULAR song from her previously submitted list. Nene sent all the demo clips she got to AGT for semi-final and finals, but the producer may not agree with them all. 

My guess : the song she will sing in the finals as instructed by AGT's producer is ALREADY IN HER DEMO LIST, meaning she need little time to practice it to perfection. And it will be a more top hit song that most AGT audiences can sing along or recognise it and at least clap along. Looking at her comfortable stance lately implys she already got it under her sleeves. No pressure. Easy as pie. May be some tricks added for a flavour.

Now it is clear why Nene Royal sang "Black Hole Sun" in her AGT QF round. It also hints at what song she will sing in AGT the finals ...