It took me awhile to get it too, so to add further, to what Eileen as stated,
You need to FIRST save ALL your packs (all green ticks) as a .cpi file. (Use the drop down menu) to a USB drive
After loading that .cpi file in Menu 2 / Pack Installation in the keyboard, all of your packs will be listed on the SX-900 under the Styles, Voices, EXPANSION/USER BUTTON.
Then any time after that, if you add more packs to Expansion manager, you have the option of again ticking green ALL of your packs, including the old and new packs, and again save them ALL as a .CPI file (old longer method). Or use the new, quicker method of ticking green all packs again, and save as a .CQI file (Corrected 8/7/20)
Bottom line, CQI just makes it quicker to load any new packs AFTER your initial CPI file was created and loaded into keyboard.
Other than that, no difference.
(Photo below shows the initial method of saving ALL of your packs as one big.CPI file for loading in the keyboard. Again, you MUST do this first, before you can use the quicker .CQI method of adding future packs.)
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