The Reference card is a card-sized guide that the player will look at whenever they need to resolve certain effects on the top of Mythos cards.
Depending on the amount of Investigators the player is playing with, the Reference will detail how many Monsters are spawned when a Monster Surge occurs, how many Clues to spawn, and how many Gate markers are placed on the board, whenever their respective symbols are shown.
The Reference is only used when the symbols on the Mythos card are used, either on the card, or with The Coming Storm Prelude, or whenever specifically mentioned. It is not used in other matters, for instance, if a Monster Surge occurs and there are no Gates that match the Omen, when only 1 gate will spawn.