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It means that the multiliners are copic safe, so they don't smudge when you use copics on top of them. If you use copics on top of a regular pen, the ink will smudge and it will look messy..

Ahh noo, that quote was from a website, sorry for the confusion.

Hahaa, yeeeah. I just couldn't understand why I had written that.. Time to go to bed I think XD