In the parlance of Scarterra reagent refers to any substance that is valuable for the purpose of creating potions, magical scrolls, or material components for certain spells. There are thousands of variations of reagents. They can come from the blood and bodies of magical creatures, they form naturally among wild plants, they can be cultivated by very knowledgeable herbalists, and they can manufactured artificially in alchemists' laboratories. Practitioners of different magical traditions sometimes use similar reagents and sometimes they do not. The differences matter a lot to Scarterrans, but to readers Scarterran fiction or players in Scarterran RPGs, it's too nit picky to get into the details. Hence we just call these things "reagents" and call it a day.