I don't know what other people mean by amendments. To me, it could be two things. One, when I clean out chicken poop that is pretty pure (not many shavings or other bedding) it is a "Green" and really good to add nitrogen to the process. I just dump that stuff on my active compost pile. I have two piles, one that is active and one that I am gatheriing stuff to start. So adding stuff high in nitrogen might be considered an amendment.
The other possible amendment for me is to add microbes to start the process. You can buy stuff that contains microbes, but I just add a couple of shovelsful (is that a word?) of compost from my last batch when I start a new one. That has the microbes in it that you need to get a good start.
Maybe somebody else has different ideas for what they consider an amendment?
Anything that increase the quality of your soil and brings it closer to the ideal of a sandy loam in the neutral PH zone (for most garden crops) could be looked on as an amentdment. For most gardeners that's compost and manures.
I think an amendment is referring to whatever you have to add to the soil that the soil is lacking. Whether it fertilizer or compost or Lyme. Its a question of "what do you need"? I don't think there is a specific list of what qualifies as an amendment.