there used to only be 3 grades of components, entry, mid, and race. and that was from each company, shimano, campy, etc. now there is many tiers in each type of component for each type of bike and multiple per manufacturer. like the shimano road has 4 or 5 types and each type has multiple grades from entry, mid, high, race, etc.
you have to google the spread sheet on what you want to make sure you also get the proper length of derailleur to go with your shifters since they make short, long, and mid and some have different pull ratios so the shifters don't work with everything properly.
as you can guess, i found out the hard way when i broke my rear derailleur. race grade derailleurs are short while the others range from short, mid, and long. the shorter the derailleur, the faster it shifts when you click or turn the shifter.