General concepts and meta information. Most of this is akin to information findable in most rulebooks, like coins of the world, languages, how magic works, and even laws of the world and its regions.
A beginners introduction to the Call of Cthulhu 7e Ruleset
An explanation of magic and how it is perceived in the Dungeons and Dragons 5e ruleset
A guide for those playing in the Blades of Adlak campaign setting, set c.4126 of the Medieval Era