The XS offers approximately 6000 different arpeggios, plus there are four independent arpeggiators that let you apply one, say, to a drum voice, one to a bass, one to a guitar, and one to a synth part. SF1
1) One arp assigned to a drum beat
2) One arp assigned to a bass groove
3) One arp assigned to a strumming pattern
4) One arp assigned to an expression motif (e.g. the flute 'flutter')
SF2
1) A different drum beat
2) A different Bass groove
3) The same strum (or different, it's up to you)
4) A different flutter

addition to arps that function like those on a Motif ES, the XS has arps that have chordal intelligence, which lets them do many things that were not possible on other Motif models such as Performance Control over mutes, hold settings etc. Many of these arps also possess up to 8 velocity zones.
Assigning these multiple arpeggios, plus being able to ‘voice' them in real time is what creates the unique performance-orientated experience of playing the Motif XS.