I've just noticed that, in the John Melas performance editor, the arpeggiator has six tabs labelled SF1-SF6. What are they for? I can find nothing in the manuals.

The only mention of them (and several other “hidden” features) is in the Data List pdf.

Item listed in grey background in the MIDI DATA TABLE area of the Data List (Page 57-66) are parameters found in the MX engine that are only accessible via MIDI. The MX is truly derived from the Motif series and shares most of the same pedigree. Indeed you can assign various Arp Phrase to be selected by MIDI commands.

The concept ten years ago was to develop apps that could run on portable devices... at one time there was a Yamaha Performance Editor Essential App for iOS devices, it supported the Motif XF, Motif XS, S90XS/S70XS, MOX, and MOXF. It allowed you to edit basic functions like Arp Phrase Selection, managing Effects, Note Ranges, EQ, and even had a unique XY Pad function... where you could assign 5 parameters to a zero-gravity track ball which would morph parameters in real-time.

Not sure if, or why this was not extended for the MX (perhaps the MX88 development grabbed that momentum) — had it been developed/extended to include the MX, it would have given access to many of the same features found it its siblings. The MX engine is capable... when addressed via MIDI.

EXTRA CREDIT: Hidden Features
The Melas Performance Editor gives you access to many features not available from the front panel. MIDI Channel assignment for example. If you wish to build a sound with several Parts sounding, simultaneously, you can do so by setting the MIDI Receive Channel for the Part (this parameter is not available from the MX front panel.

The Data List booklet will outline them — and they are usually available via the Melas Editors. If you write Apps you’ve got everything you need!