SFI is this silly foundation that supposedly tests race equipment, and makes sure they meet their standards.
When you race at a sanctioned track, certain equipment at certain speeds must be SFI certified. Stuff like balancers, tranny shields, seat belts etc.
To find out what you need and when, pick up a rule book.
Personally, it's a big money grab if you ask me since some of the stuff "expires", then you have to buy a new one, or get it recertified....