Hi Pietro,
when Expansion content ist installed on the keyboard (using the "Pack Install" function and the .cpi or .ppi file created before with the "Save as Pack Install File" function of the YEM), the following happens:
The Wave samples of the Expansion Voices are automatically saved in the internal Expansion Wave memory (which has a size of 1024 MB on the SX900). This memory is basically of the same type as the Wave memory in which the samples of the Preset Voices are stored (so-called "NAND Flash Memory", a type of memory from which the
data can be read out very quickly). The only difference is that the Preset sample data CANNOT be changed by the User. The installed Expansion Voices can be used
in the same way as Preset Voices, but they are not accessed via the
Preset button in the Voice Selection display, but via the
Expansion button (as already mentioned by Eileen before).
All other contents of the individual Packs (except for the Wave samples) are automatically copied into the corresponding subfolders of the Expansion folder during the Pack Installation. (The Expansion folder is actually in the User drive. But you can only access it via the Expansion button in the file selection displays, NOT via the User button.)
While the Pack Installation, a
.vce file is also automatially created for each Expansion Voice. This file is automatically copied into the respective Pack subfolder in the Expansion folder. These .vce files will later be shown in the Voice Selection display when you press the Expansion button there and select a Pack folder. So they are used to
call up the Expansion Voices. The Voice Set data required to (automatically) generate these .vce files come from the .
uvf files contained in each Pack file (.ppf/.cpf). These .uvf files (which are programmed in "XML") are not visible to the user. In the case of
unprotected packs (.
ppf), however, they are directly available as individual files in the YEM User folder ("C: \ Users \ <YourUsername> \ AppData \ Local \ Yamaha \ Expansion Manager \ Packs \").
I hope my hints help to bring some "light into the dark" ...

Best regards,
Chris