It is also possible to tie notes together and place rests in the arp patterns. The way arps fit into the XL-1's programming is that there are both a master arp and preset arps. In the Edit menu, a 'preset arp' can be selected, to tie an arp pattern to a preset. You might want to try some of these out to get a good idea of what arps really are and what can be done with them. With that disclaimer in mind, I'll do the best I can.
Riffs are very similar to arps, but have a function that is more like a sequencer track.
When you press the Audition button on the front panel, you are playing a riff starting at the beginning. As with arps, there is a master riff as well. However, I noticed while experimenting that a few of the XL-1's riffs do seem to have more than one part. Meanwhile, another LED blinks at each quarter note. Now, about those 16 tracks.