Hi Graham,
it's exactly the same as on e.g. all Tyros models, Genos, PSR-S/SX models: You can save your Registration Banks to any location, and you can always copy, move or rename the Registration Bank files if desired. They always will work, from any drive and any location. In terms of functionality, it makes no difference whether you save the Registration Bank files in the folder of the style used or not. As usual, I recommend using a separate main folder on the USB stick for each file type (e.g. Registration, Style, Multi Pad, MIDI, User Voice, Text) and creating sub-folders in it as required.
BUT you must not move or rename the user files that are linked in the Registrations (e.g. user styles, multi pads, MIDI files, txt files). This also applies to the folder in which the linked files are located.
Otherwise the Registration could no longer find / load the linked files, and you would have to update / re-memorize each Registration and re-save the Bank.
Best regards,
Chris