from bing chatGPT

I couldn't find a definitive answer to your question. However, based on the Yamaha XG Normal Voice List ², it seems that "K" refers to different sound effects such as "Key Scale Panning", "Slow", "Fast Decay", "Double Attack", etc. It's possible that these effects are applied to the instrument sounds with which they are associated.

I hope this helps!

Source: Conversation with Bing, 3/29/2023(1) XG Normal Voice List - Yamaha Corporation. https://usa.yamaha.com/files/download/other_assets/6/317936/QY70E2.PDF Accessed 3/29/2023.
(2) Letter 'K' in XG MIDI Voice Names - PG Music Forums. https://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=759296 Accessed 3/29/2023.
(3) MIDI Voice List Assignments. https://www.midiplayertools.com/Documents/MIDIVoiceChart.pdf Accessed 3/29/2023.
(4) Demystifying Yamaha's XG Soundcards - Sound On Sound. https://www.soundonsound.com/techniques/demystifying-yamahas-xg-soundcards Accessed 3/29/2023.
(5) XG XG/XGlite VOICE MAP - Yamaha Corporation. https://jp.yamaha.com/files/download/other_assets/9/321749/voice_list.pdf Accessed 3/29/2023.
(6) Yamaha CBX-K1XG - Sound On Sound. https://www.soundonsound.com/reviews/yamaha-cbx-k1xg Accessed 3/29/2023.
(7) Applying XG Effects to the PSR8000 Voices via MIDI - Yamaha Corporation. https://faq.yamaha.com/usa/s/article/U0002203 Accessed 3/29/2023.

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