Dolby atmos is like the new around sound, it's been called as artificial full 3d sound because sound comes from all directions, artificially; you should YouTube a vid to see how they do it.
Regarding the subscriton, no, it does not require a subscription. The code is to download an "APP" from the xbox store so the headphones work properly, it's like the software for the headphones to do their job right. So no, you don't have to pay monthly or anything, just use the code in the xbox store to download that app and you should be good to go.
Hope I was able to help!