Hi --
Chord Step Record is a feature of MIDI Multi Recording. Basically, one enters chords into a Chord Track. Then, click the Expand button to create the accompaniment. Save everything in a MIDI file. That's it, really.
Chord Step Record has many uses for songwriting, practice, and so forth. This feature is the main reason why I got into Yamaha arrangers!
"Step Recording/Editing Songs (Step Edit)", SX900 Reference Manual, page 77. Chords, specifically, on page 81.
Hope this gets you started -- pj