So, I understand the RecArp function.  I can record arpeggio-triggering notes, and I can record the arpeggio output as normal notes.  How does the Motif “know” which notes are which, when played back?  (And, does it somehow know which arpeggio to play?)

Following this, how does one “convert” previously-recorded arpeggio-triggering notes to a bunch of regular notes, as if I had set RecArp to “on”? 

An example of why I’d want to do this is to sketch-out chord progressions separate from the actual strumming rhythms for instance, and then “bake” them for further edits once I’ve settled on a chord progression I’m happy with.  But on second though, I think this would only be useful if Motif can let you specify different arpeggios throughout a song for the current Part it’s enabled for - and I’m guessing you can’t, which would make arpeggio-triggering-notes less useful.  I hope I’m making sense.

Although these don’t all directly apply to the ES, they should help in understanding differences between the various Arp Key Modes:
http://www.motifator.com/index.php/support/view/arpeggio_key_modes
http://www.motifator.com/index.php/support/view/recording_midi_data_generated_by_an_arp_with_key_mode_sort_direct
http://www.motifator.com/index.php/archive/viewthread/318860/

Thanks, but none of these really answer my question. The Key Modes explains how an arpeggio can respond to playing the keys, the second one seems to explain how to record a complex arpeggio with multiple key modes, and the 3rd one seems to just be a clarification of the first one.

I’m trying to understand how a song’s sequence data “knows” which notes should trigger an arpeggio (which can be modified globally) and which should not.

You can choose whether the sequencer records “direct” notes (the ones you play) or “sorted” notes (the arpeggiated ones), depending on RecArp and KeyMode settings.

Notes are stored in the sequence as note-on and note-off events - whether those notes were just the keys you played, or generated by the arpeggiator.

Perhaps Bad_Mister‘s post in this thread will help to clarify this:
http://www.motifator.com/index.php/forum/viewthread/454611/

The sequence data “knows” nothing about arpeggios, but it can be used to turn the Motif’s Arps ON/OFF:
http://www.motifator.com/index.php/archive/viewthread/331516/