I could paste a huge chunk from p. 51 of the Owners Manual, but I encourage anyone interested to download it and read it for yourself. Yes, the feature is limited to certain styles. I'm guessing that these styles have a subtle and logical progression across the four variations. The user can limit how far the arranger wanders from the "base" selected variation.
Remember that first and foremost this is a piano. I'm guessing that the "more notes" feature is optimized for full keyboard or piano-style chord recognition. In two-handed scores there are always some passages which are more dense than others. I would guess the arranger engine looks at how many keys are pressed on a beat, or are being held down or sustained.
The Adaptive Styles seem like a great idea, but so much depends on the execution. I'm sure that we'll see demos on Yamaha's website, or YouTube shortly, and then we'll know more. This, and the Accent/Unison are some of the most significant features to emerge in a long time. Whether the arranger is evolving or dying, it's encouraging to see companies trying to make them more interactive!