Levels of abrasiveness.
1 is what I like to call a maintenance polish. Use prior to sealing your paint just to clean it up a little.
2 can be used for very light scratches and swirls, never requiring a follow up polish with lesser abrasives....
2.5 for more moderate swirls and scratches, may or may not need follow up with a lesser abrasive SSR, depending on vehicle make, pad selection and techniques. (It did on a Caddy, but not an Acura).
3 for sever swirls and scratching, will require follow up with 2/2.5.
All are "ok" to use in the sun, not the best route, but "ok".