Does it mean free choice ? I got confused because on X Factor USA, Alex & Sierra sang Taylor Swift song during Big Band, so it must be free choice as long as they perform with music band behind them ?

Not very much really, as the song choices rarely have anything to do with big bands.

I just look at it as a live instrumental as opposed to the usual recorded instrumental they use.

So contestants can chose any song they like ?

I think 'Big Bang' would usually refer to something old school and ratpack... however XF Big Band is basically acts singing with live instruments as opposed to a backing track.

Therefore it's basically free choice IMO

Might be a better show if they could.

In practice the songs will come from the usual pre-cleared list, most of which will already have been done to death in previous years. And the contestants will end up singing whatever they're told to...or else.

It results in ghastly spectacle of skinny yoofs getting lost inside a man's tuxedo and wearing a loosely knotted tie. Probably wearing trainers instead of nice shiny shoes too. Generally its a crap and pointless theme that they trot out if Buble or Robster have a swing record they are trying to plug on the live slot on Sunday.

It is basically performing with a live band behind them rather than a backing track. Technically it could mean that they do old Rat Pack/Swing style music but they rarely do.

Michael Buble is the king of performing with a big band in recent years. He flows with the band rather than trying to sing over it, and the fact his voice is beyond powerful also helps

Also for anyone who has seen him live, his finale for each show is usually Accapella when the big band stops playing but he also drops his mic and just sings without one and the whole arena goes quiet to listen to him fill a 15,000-20,000 seat arena without a mic. Quite a sight to see.

They have to perform classics from bands with a BMI in the obese + range.

That's Big Waistband week - week after next.