Hi Nova227,
.sst means Session Style.
Here is a list of the Style file extensions used by Yamaha:
.sty .STY = Standard Style
.bcs .BCS = Basic Style
.prs .PRS = Pro Style
.sst .SST = Session Style
.fps .FPS = Free Play Style
.aus .AUS = +Audio Style
.pst .PST = Pianist Style
.pcs .PCS = Piano Combo Style
.scp .SCP = DJ Style
By the way, the ".sst" file extension, as you've already noticed, is used by Windows for other purposes. You can just ignore this. If you want to open style files (e.g. .sst) in Windows by double-clicking in a specific program, simply right-click on the relevant style file, select "Open with..." and then the desired program/app. It is important that the check mark "Always use this app" (or similar) is set. I have linked all style file types with the vanBasco player in order to be able to quickly listen to the pure MIDI data of styles.
Best regards,
Chris