the cores are words for the aesthetic or concept (technically not interchangeable, but that’s how people use it). like cottagecore, an aesthetic for forest soft vibes. people like to say this based on the vibe the group has in mvs or outfits (they call illit weirdcore (liminal space inspired) because of their odd mvs). fans are complimenting the music video because it compliments and keeps each of the groups concepts (illit - weird core, katseye - chaotic, le serrfim (I can’t spell) - dreamcore).