the different grades are for different levels of contamination, a car with light contamination will need a fine or ultra fine clay and a car with heavy contamination ie lots of tar spots,heavily bonded contaminants,fall out,grit,overspray etc will need a more aggressive clay to remove them.
if you havent clayed before id recomend a fine or ultra fine clay, aggressive clay in the wrong hands will lead to marring and re-scratching of the surface.