I've stumbled on this phrase in Terry Pratchett's Lords and Ladies:
“This is the Moon,” Diamanda repeated, “for those who weren’t paying attention.” She held up the card. “And what do we see here—you, Muscara?”
“Um…it’s got a picture of the moon on it?” said Muscara (née Susan) in a hopeful voice.
“Of course it’s not the moon. It’s a nonmimetic convention, not tied to a conventional referencing system, actually,” said Diamanda.
I know what mimetic and convention mean, but I'm not sure what nonmimetic convention means in this context. Any help? Thanks.