There IS no fill button...

Just different ARPS, which some of the presets (not many) have been programmed as fills. If I remember right, some of the blues setups, or I can't exactly remember WHICH ones, but there are definitely some set up this way.

The XS is NOT an arranger, and it's current OS can't make it into one. Each ARP is a complete thing, and either has to be played one after another (press another ARP and as soon as the current one finishes the loop, it goes into the one you just pressed (it waits for the end) or it can go immediately, but it doesn't remain in sync. At least from trying most of the factory presets, this is the impression I got. If it CAN drop into a fill in sync, at any time, there are NO presets that do this, something I find hard to believe Yamaha WOULDN'T have done.

So every move must be planned ahead, rather than be interactive, like an arranger.