Hi Mark,
registrations are brilliant.
You can setup a style for a song exactly how you want , voices , variations, chord loop , multi pad etc etc what you can save to to a registration is pretty mind boggling.

Being a home player, I never bothered to create a song book on any of my korgs,
I’d get out my sheet music, pick some styles to try, then jot down which one worked best. Change a voices, etc , save it as a user style with the song’s name. Some styles were ok as they were. Then I’d just write which style to use on the top of my sheet music.

The way I now have my p121 ( as controller )and sx900 set up, I need to use Registrations to get me thru a song. Never been an organ player, so I can’t get used to the stacked system. I basically have my piano in front, sx to the side in L shape configuration. I can still see screen and reach sx buttons, but pretty much impossible to press buttons in the middle of playing a song.
So this is where the registrations come into their own. I usually have at least 4 set up one for each of the variations.
Then there is a function called Registration Sequence. Here I put in the sequence in which I want my registrations to play back.
I assigned one of the pedals ( which I can reach) to this function.
Every time I tap it, it goes to the next registration without me having to touch the SX. Ie I can set it to go from registration 1 to 2 to 3 to 4 and loop back, or if the song requires I could get it to sequence thru from 2 to 1 to 4 back to 3.
Whatever I want. It follows the sequence I have put in.
Sounds like a lot of work, but , really simple and takes a few second and it gets saved.

I’ll probably end up doing playlists. Don’t have that many songs together at this stage, have to decide whether I’m better off doing it thru my Song Book app or via Playlist function.