Hi bernamato --
Judging from your question, you probably know the basic difference between the Montage/MODX arpeggios vs. Genos/PSR styles. Even though Montage/MODX aren't arrangers, it's possible to make Scenes which consist of coordinated arpeggios. Switching Scenes is similar to switch Style sections, but no auto-fill, auto switching from intro to main, etc.
So, it sounds like you're wondering about how to make Montage/MODX arpeggios? That's a fairly deep topic and I recommend reading the arpeggio-related sections in the Montage/MODX reference manual. Then, search for "how to make arpeggio" messages on
www.yamahasynth.com.
To get directly to your question, MIDI data is converted to an arpeggio in one of three ways (Convert Type):
Normal (Normal Arpeggio) The Arpeggio is played back using
only the played note and its octave notes.
Fixed Playing any note(s) will trigger the same MIDI sequence data.
OrgNotes (Original Notes) Basically same as Fixed with the
exception that the Arpeggio playback notes differ according to
the played chord.
There is also the Key Mode parameter, but I think you're asking about the OrgNotes conversion above.
When the Convert Type is OrgNotes, there is one other parameter: Org Notes Root. All the rest of the CASM-like stuff is hidden. That's the up-side.
On the down-side, there is still a steep learning curve for arpeggio creation since there are many other parameters which are not a concern in Styles because Styles are not arpeggios. :-)
Hope this info helps -- pj
P.S. I play both MODX and Genos, and have tried converting between styles and arpeggios. It's a lot of work in either direction.
https://www.yamahasynth.com/learn/montage/mastering-montage-arpeggio-making-101-part-ihttps://www.yamahasynth.com/learn/montage/mastering-montage-arpeggio-making-101-part-ii