Each and every card has an alignment. It is either part of a faction (Darkness, Forest, Mountains, Plains, Swamp) or is Neutral.
Extra damage between factions is illustrated below:
“- Soul catcher (which is not unique because you have e.g. Soul Catcher 1 on Zendar Shield ) ...Eternal Darkness
“- Dark assassin Neutral cards aren't part of any specific faction so they can be used by all heroes.Neutral
“- Plains assassin Most Healer cards are of the Forest alignment and so are practically all...Forest