A single part performance is one that only contains one part - ie the other 15 parts of the performance are empty slots. That's not to say though that that single part is only one sound. Each part can have a maximum of 8 elements with each element being an individual waveform in an awm2 part so in theory a single part could actually be a combination of 8 sounds. I'm not at my Montage to check but I would imagine the parts you mentioned fit into this category - eg some elements of the Piano and Strings part will be for the piano sound, the others for the string sound. In fact those sounds are part of the "legacy" sounds present in the XS and XF boards which were Voices in the Motif nomenclature - these have all become single part performances in the Montage.

As the individual elements of a part are themselves editable there is no reason why you can't edit the individual "sounds" of this dual sound single part performance - eg the balance between the parts can be altered by adjusting the amplitude of the elements.

By extension you can also do the reverse by adding the elements of two separate parts together into one part (as long as there is room for the combined elements) though the drawback of this is that the combined part will only have access to 2 insertion effects rather than 4.

OK Phil, thank you. Experimenting with 'Piano & Strings' I find it has 4 arpeggio parts assigned to it, and plaaying around with those I managed to assign a fifth which is rubbish. To my horror, I can't get rid of it! No 'Erase' or 'Delete' function available ... by tapping the offending item, I can change it for another, but how do I get rid of it altogether, please? Teach me to fool around ...

Do you need to delete it? Each of the 8 possible arpeggios for a part are assigned to the 8 Arp Select buttons on the right hand side of the keyboard. They will only play if you hit the corresponding button so having an extra arpeggio isn't a problem as only one can play at a time.

Experimenting with 'Piano & Strings' I find it has 4 arpeggio parts assigned to it, and plaaying around with those I managed to assign a fifth which is rubbish. To my horror, I can't get rid of it! No 'Erase' or 'Delete' function available ...

On the PART Edit screen where you assign Arp Phrases, highlight the Arp name, a pop-in from the left appears with option to Search or Number.

Touch "Number"
Enter "0"
Touch "Done"

Thanks guys! The only way I can't screw up is to never turn it on ... I have to work on the principle that you can't do any real damage (I hope!) and if I got in, there must be a way out ... yeah, right! Praise be there are people about Who Know What They're Doing ...