I’m imagining contextual details in a fantasy fiction setting like technical drawings of alien plants, diagrams of demon physiology, guidelines for communicating with inter dimensional beings. I suppose you see it a lot with occult fiction (finding information in dense intricate tomes,performing precise rituals, etc) and in fantasy where the author establishes realism by conceiving of the types of things you might find in a scholar’s library in that setting.
Is there a general term for that type of documentation of a certain lore, mythos, or any body of knowledge (whether or not connoting fictional contents)? “Marginalia” has the spirit of what I’m imagining.