Note: Ratings are built-in if your discipline is set to Hunter Jumper, you won't see the Ratings menu item if your discipline is Hunter Jumper. They will be pre-configured as AA, A, B, C, and NR.
Ratings are only required at all if you use increment point tables by division and rating, if are not using those you do not need to set up ratings, otherwise you absolutely do need to set them up.
This is basically a list editor. You just need to add the ratings to suit.
The "Not Rated" item is built-in, you won't be able to delete it or move it from the top spot, it serves as the default rating.