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Off-Model is the lifeblood of Fanart. The ability to draw whatever you want using a established design as a base. To choose whatever headcanon you want with the character. Off-model theoretically solves all "legality" debates because any and all characters can be headcanon as 18+ and drawn to look as such with no worries for coomers(sure, that won't stop "canon" pedohunter crybabies from seething, but still). That's what makes Fanart so big and popular, today. The only people who need to adhere to the "On-model" rule are the artists who are being paid by official sources to produce any kind of material for the brand.